The study focuses on the analysis of inherent optical properties of lake waters characterized by consisting of several optically active substances (OAS) (phytoplankton, suspended particles and dissolved organic matter). The results obtained can be useful for deriving the algorithms of optical remote sensing models which need the spectral data on inherent optical properties of the aquatic environment. The spectral absorption and attenuation coefficients were measured in situ using the instrument ac-9 for eight wavelengths in the range of 400–700 nm. The investigation sites were five Finnish and six Estonian lakes. The concentrations of chlorophyll a and total suspended particulate matter, as well as the absorption coefficient of coloured dissolved organic matter (at 380 nm) were determined in a laboratory from water samples taken concurrently with ac-9 measurements. There are three main objectives: (1) to calculate the values of absorption and specific absorption coefficients of tripton on the basis of ac-9 and laboratory data; (2) to estimate the contribution of pure water, chlorophyll a, dissolved organic matter and tripton to the total absorption and attenuation coefficients (results for ac-9 wavelengths and also for the PAR region, 400–700 nm); and (3) to determine the slope (and its variability) of the scattering coefficient spectrum for each measurement series and to estimate the reasons of this variability. At the wavelength of 532 nm the values of the specific absorption coefficient of tripton varied between 0.013 and 0.098 L mg−1 m−1, while the slope of its spectrum ranged from 0.0060 to 0.0109 nm−1. The contributions of different components of the aquatic medium to the total absorption/attenuation coefficients varied noticeably from lake to lake. The slope of the scattering coefficient spectrum was in the range of 0.32–2.5, the multiple regression between the slope and three main optically active substances jointly gave the determination coefficient R2(adjusted)=0.655.