This collection comprises physical measurements of the water column and surface waters, together with supporting discrete chemical and biological datasets. The data were obtained from the Irish Sea and in the sea off western Scotland over 4 periods: 17 and 23 August 2011 and 06 - 07 March 2012, all collected on Seiont IV cruises and 15 - 22 June 2012 obtained using the RV Prince Madog. These datasets and their collection methods are as follows: 1) LISST particle size data - A LISST 100X type C laser diffraction instrument was lowered in a frame from the ship and the depth-averaged volumes of particles in 32 size classes in a water column from the surface to a depth of 10 m (or the bottom, where shallower) were measured. 2) CTD profiles of conductivity, temperature, sigma-theta and salinity. At each station, a CTD with attached rosette was lowered, with data measurements taken. 3) SPM, mineral SPM, chlorophyll and CDOM water sample data. At each station a surface water sample was collected either in a bucket or in a rosette sampler on the CTD and triplicate sub-samples were filtered and subsequently dried and weighed, baked (at 500°C for 3 hours to remove organic material) and weighed again. 4) CDOM discrete samples taken from CTD and underway. Surface water samples collected at each station were filtered through 0.2 μm filters and the spectral variation of the absorption coefficient of the dissolved material in the filtrate was measured in a 10 cm cell in a Shimadzu 1600 dual-beam spectrophotometer, using distilled water as a reference. 5) Water column inherent optical property profiles. Measurements of beam attenuation were made using a Sea Tech T1000 transmissometer (20cm pathlength) fixed to the CTD on the RV Prince Madog. At some stations, vertical profiles of downwelling irradiance and upwelling radiance were made with a PRR radiometer. These cruises formed the fieldwork component of the NERC-funded project "Measurement of the abundance and optical significance of sub-micron sized particles in the ocean". The project aimed to use different magnifications and commercially available in-situ particle sizing instruments to create a package of instruments for measuring the undisturbed particle size distributions from 0.2 μm to 1 mm. This package will first be used in a turbulence tank to 'film' the flocculation process. The insight this gives will be used to construct new theoretical models of the particle size distribution. Because the camera also measures the shape of the particles, differences between observed and calculated optical properties can be compared, for the first time, to particle shape. Finally, the complete dataset will be collated to determine what size particles, under what conditions, are primarily responsible for the signals seen in visible band satellite images of the oceans. The NERC-funded project was held under lead grant reference NE/H022090/1 with child grants NE/H020853/1 and NE/H021493/1. The lead grant was held at Bangor University, School of Ocean Sciences by Professor David Bowers and ran from 01 April 2011 to 31 March 2014. Grant NE/H020853/1 was held at the University of Plymouth, School of Marine Science and Engineering by Dr. William Alexander Nimmo Smith and ran from 01 October 2010 to 30 September 2013. Finally, grant NE/H021493/1 was held at the University of Strathclyde Physics Department by Dr. David McKee and ran from 01 April 2011 to 31 March 2014. All data have been received by BODC as raw files from the RV Prince Madog and Seiont IV, processed and quality controlled using in-house BODC procedures.