The data are activity concentrations of Tc-99 and Se-75 in 61 species of flowering plants after exposure under similar controlled conditions. Species selection was focussed on a hierarchical, taxonomic spread across the herbaceous flora of the British Isles. Individual plants were grown in pots, with supplementary lighting and heating, until they reached the exponential phase of their growth. They were then radiobelled by addition of 109kBq/L Tc-99 and 95kBq/L Se-75. After 48h exposure, aboveground biomass and compost samples were taken and analysed for Tc-99 and Se-75. These data helped refine estimates of Concentration Ratios for use in radioecological modelling and were generated as part of NERC grant number NE/L000342/1. Full details about this nonGeographicDataset can be found at https://doi.org/10.5285/e5122291-b5e2-47de-bf14-db67125f4c14