Sensitometric properties were measured for a positive diazo-type photoresist at wavelengths of 365, 405, and 436 nm. It is shown that the exposure data at these three wavelenqths may be combined according to Van Kreveld's additivity law to accurately predict both simultaneous and successive exposures made with combinations of these wavelengths. Exposure modeling using Beer's law and first order photolysis kinetics, when combined with the measured sensitometric properties, supports the validity of Van Kreveld's law for simultaneous and successive exposures. The combined modeling and experimental data also support the existence of a critical inhibitor concentration that defines complete exposure and is the same for all wavelengths as well as for monochromatic simultaneous and successive exposures. Applications of characteristic curves of exposure depth as a function of exposure are outlined.