Nanostructured silica-titania composites (STCs) were casted as large cylindrical pellets to be used in an annular packed-bed reactor. Their performance for the photocatalytic conversion of toluene was investigated as a function of relative humidity (RH) and space time. The 172 angstrom STCs were analyzed for their titania surface area (SA) available for reaction. Using silica as the titania support resulted in a loss of available titania SA of about 20%. However, casting the STCs in a large cylindrical pellet shape did not seem to further decrease the available titania SA. The effect of RH in the conversion of toluene was very significant. By increasing the RH from 13% to 90%, the conversion of toluene increased by 57%. Despite the large increase in RH, the catalyst still showed the yellowish coloration found at low RH. which is usually associated with deactivation. Toluene conversion increased with space time and steady-state conversions as high as 93% were achieved by increasing the space time up to 25s. For all conditions studied neither external nor internal mass transfer resistances were found to be significant. The pseudo-first-order kinetics equation was successfully fit to the data and resulted in a rate constant of 0.12 s(-1). (C) 2010 Elsevier B.V. All rights reserved.