In situ measurements of downward irradiance and upward radiance, together with determinations of surface concentrations of chlorophyll, suspended particles and colored dissolved organic material (CDOM) were made at 126 stations during 5 cruises in the Irish and Celtic Seas. Twenty one of these stations provided acceptable match-ups with SeaWiFS ocean color images. Normalized water leaving radiances for the selected stations derived from SeaWiFS data were well correlated with those measured at the sea surface (r2 = 0.92). In situ observations from the whole region, which covered a wide range of water types including turbid coastal regions and a coccolithophore bloom, were therefore used to test the performance of SeaDAS algorithms for surface chlorophyll concentration (Chl) and the diffuse attenuation coefficient at 490 nm (Kd490). In general, Chl was over-estimated and Kd490 under-estimated by SeaDAS. However the algorithms performed acceptably in some regions such as St George’s Channel (r2 = 0.70 for Chl, r2 = 0.74 for Kd490). Examination of SeaWiFS and AVHRR images of this region for July 2002 revealed complex patterns of surface temperature and chlorophyll with a high degree of spatial correlation. The highest chlorophyll concentrations occurred on the stratified side of the Irish/Celtic Sea front.