Computation of phytoplankton pigment concentrations from the radiances received by the coastal zone color scanner (CZCS) is a complicated process. Water-leaving radiance, a small portion of the total signal, controls the pigment content, but fully 80–90 per cent of the total radiance is due to Rayleigh and aerosol scattering. These effects must be accurately modelled if reasonable pigment concentrations are to be computed. Rayleigh radiance is relatively easy to determine from either single or multiple scattering algorithms. Several examples of the effects of single versus multiple Rayleigh scattering on pigment content are given. The aerosol component varies both spatially and temporally and is poorly determined. Historically, the ratios of aerosol radiance in channels 1–3 to that in channel 4 have been assumed to be constant for an entire image. The proportionality constants are found by assuming values of water-leaving radiance for low pigment waters and theoretically should be independent of aerosol magnitude. However, a single ratio often is not representative of an entire image and a strong correlation may exist between aerosol magnitude and the aerosol ratio, especially when Rayleigh radiance is computed from multiple-scattering theory. These variations can produce large differences in derived pigment concentration. Hence aerosol ratios generally may not be reliable estimates of aerosol radiance. These considerations must be addressed before accurate pigment concentrations can be obtained from an arbitrary CZCS image.