Precise kinetic studies of photocatalytic reactions in solid catalyst water suspensions require the accurate description of the radiation field-light distribution-inside the reactor. Solution of the radiative transport equation (RTE) inside the reaction is one of the best ways of accessing to such information. For solving this equation, a minimum of two parameters (the absorption and scattering coefficients) and one scattering spatial distribution function (the phase function) are needed. These attributes are directly associated with the optical behavior of the reacting system and are not independent of catalysts more conventional properties. A complete report on the physical and optical characteristics of titanium dioxide particulate suspensions in water is presented. Results were obtained for six different commercially available powders. The investigated parameters were (i) size of elementary particles, (ii) size of particle aggregates in water suspensions, (iii) specific surface area, (iv) spectral extinction coefficient, (v) spectral absorption coefficient, and (vi) spectral scattering coefficient. The last three were obtained as a function of wavelength in the range 275-405 nm. All measurements were made following a standardized protocol for the preparation of the solid suspensions. Scattering and absorption effects could be deconvoluted from the extinction coefficient by applying a very simple radiation transport model to the analysis of the experimental data. Experimental information was obtained by means of specially designed spectrophotometric measurements made with conventional cells, combined with results obtained with an integrating sphere accessory operated in the transmission mode. These properties-particularly the optical ones-are required to solve the RTE and (i) to calculate precise values of photocatalytic reaction quantum yields and (ii) to fully characterize radiation energy absorption effects in the kinetics of photocatalytic reactions. Moreover, these data are indispensable for devising scaleup procedures in photocatalytic reactor design.