A controlled experiment was set up to study the reflectance spectra of phytoplankton cultures isolated from marine and fresh waters in Singapore. Typical reflectance of chlorophyll was observed in the green band (540-570 nm), whereas absorption was mainly in the blue band (400-500 nm) and red band (650 nm). Additional reflectance peaks were observed in the near-infrared (NIR) region at 687 nm, 719 nm and 760 nm. A comparison of the spectral profiles at high and low chlorophyll concentrations indicates that there is a significantly greater increase in the overall reflectance in the (NIR) band compared to that in the visible band. Colour ratios and first derivatives at selected reflectance peaks were correlated with chlorophyll a content. The red-NIR ratio and first derivative at 687 nm (NIR) give the best correlations with chlorophyll a . When marine clays were added to the pure algal culture, a preferential increase in reflectance of light at certain wavebands was observed. When natural marine clay was added, there was a greater increase in the visible band, especially the green wavelengths, compared to that in the NIR band. In contrast, when combusted marine clay (organic content removed) was added, a relatively greater increase in reflectance in the NIR band was seen. The experimental results were able to partially explain the trends observed in spectral reflectance measurements of the coastal waters of Singapore.