A computer simulation study, using the Monte Carlo calculation procedure, has been carried out to determine in what way the relationships between the apparent and the inherent optical properties of natural waters are affected by the angle of incidence of the photons on the surface. With the particular normalized volume scattering function used in these simulations, the vertical attenuation coefficient for downward irradiance at the midpoint of the euphotic zone, Kd(zm), can be expressed as a function of the absorption (a) and (b) coefficients, and the cosine of the incident photons just below the surface (µo), in accordance with urn:x-wiley:00243590:media:lno19842920350:lno19842920350-math-0001 Thus, as the direction of the incident light departs increasingly from the vertical, Kd(zm) increases but becomes progressively less responsive to increases in scattering at constant absorption. The irradiance reflectance just below the surface [R(0)] at all angles of incidence increases linearly with the ratio of the backscattering coefficient (bb) to the absorption coefficient, and at any value of bb/ a increases as the angle of the light departs further from the vertical, in approximate conformity with urn:x-wiley:00243590:media:lno19842920350:lno19842920350-math-0002