On site measurements of pH, conductivity and temperature for thirteen sites in the Humber catchment and three sites in the Tweed catchment within the period 1994 to 1997 as part of the Land Ocean Interaction Study project (LOIS). Sites were sampled at regular weekly intervals and more intermittently during high flows (on average an extra sampling once a month per site). Samples were obtained using a wide neck PTFE bottle in a plastic covered bottle carrier (lowered from bridges where possible otherwise collected by immersing sample bottle by hand in the water as near the main flow as possible). pH and conductivity reading were taken using Mettler Toledo Check Mate meters and probes. Temperature readings were obtained using a PT 100 probe with a Digitron 3204 Pt meter (RS components). The measurements were carried out by members of the field sampling team at York University, as part of the Land Ocean Interaction Study (LOIS). Full details about this dataset can be found at https://doi.org/10.5285/329cbe8f-a292-4c5f-a15a-6528774dbee7