Optical backscatter sensors (OBS) have been used to estimate in situ concentrations of suspended particulate matter (SPM) in aquatic systems for about 20 years. The logical next step is to use the differential responses to varying sizes and composition of the particles across a range of wavelengths to characterize discrete mixtures of sediments. To do this, we examined the response of the backscatter coef®cients in the optical range 442±671 nm to de®ned mixtures of suspended quartz and carbonate sands in a turbulent jet. We found positive linear responses of the backscatter coef®cient to the amount of suspended sand at all wavelengths. The regressions differed among the wavelengths for mixture constituents, but analyses of error propagation indicated that concentration prediction using mixture component calibration curves was problematic. When the colour of the sand was changed, the response of backscatter to concentration at some of the wavelengths changed and errors in prediction of mixture concentration were low. Thus, the differential optical backscatter response of sediment components can provide a means to estimate suspended sediment concentrations in the ®eld from individual calibration equations when the components are different colours. Under certain conditions, this approach can be used as an alternative to the traditional calibration procedure which uses sediment collected from the sampling site during the OBS measurements.