This paper demonstrates the influence of the relaxation time of photoexcited charge species on the photoluminescence and photocatalytic activity for oxidation–reduction reactions by CdS nanostructures of different dimensions. CdS nanospheres (size ∼10nm) and different aspect ratio (17 and 23) CdS nanorods have been prepared by two different techniques. CdS nanorods formed by autoclaving is found to be more lengthy, wider (length ∼170nm and width ∼10nm) and having better crystallinity than CdS nanorods (length ∼126nm and width ∼5.5nm) prepared by refluxing under similar conditions. Characterizations have been done by optical absorption, fluorescence emission, time resolved fluorescence decay, scanning and transmission electron microscopy, X-ray diffraction and BET specific surface area measurements. Relaxation lifetime of photoexcited electron–hole pairs is measured to be 20, 24 and 116μs for CdS nanosphere, shorter and longer CdS nanorod, respectively, seems to be responsible for the observed fluctuation in photoluminescence and photocatalytic activity. The photooxidation rate of salicylic acid (0.5mM) and photoreduction of p-nitrophenol (0.2mM) are significantly improved with increasing dimensions of CdS nanorods despite having a comparable surface area (81 and 76m2g−1) and CdS nanospheres (18m2g−1) exhibit poor photocatalysis. The better delocalization of charge species along the radial as well as longitudinal dimensions of CdS nanorods, higher crystallinity and delayed recombination time facilitate electrons or holes for active participation in the photoinduced reactions, and Au deposition always displayed superior photoactivity.