The increased complexity of phase–modulated spectroscopy compared with incoherent light techniques of near–infrared spectroscopy is justified if measurement of path length is necessary. In order to assess the variability of optical transmission in the head of adults, 96 subjects of varying age, gender and skin pigmentation were studied using an experimental three–wavelength time–shared phase–modulated spectroscopy device. The optical path length was measured at each wavelength, and saturation and haemoglobin–free path length were calculated. The path length varied linearly with the separation of the optical probes, but gender, age or skin pigmentation were not associated with differences in path length. Haemoglobin saturation averaged 68 per cent and varied with age in a non–uniform fashion. Haemoglobin–free path length varied between genders, being 8 per cent longer in women than in men. Measurements could not be performed at 5 cm due to optical attenuation in 36 per cent of subjects These subjects were more often young, but they were not otherwise distinguished by gender or pigmentation differences. Subjects in whom measurements could be obtained at 5 cm had longer haemoglobin–free path lengths than did subjects in whom optical attenuation prevented these measurements. These data confirm the occurrence of significant differences in optical transmission in adults and support the use of phase–modulated spectroscopy techniques that measure path length.