Interest in porous silicon (pSi) (and, more broadly, silicon nanoparticles (NPs)) has increased along with their concomitant use in various commercial and consumer products, yet little is known about their behavior in the natural environment. In this study, we have investigated the photosensitization, optical, and surface properties of pSi as a function of time in aqueous systems. Samples were prepared via an anodic electrochemical etching procedure, resulting in pSi particles with diameters of ca. 500 nm, composed of a porous network of Si nanocrystallites of 2-4 nm. Initially, pSi particles generated significant amounts of (1)O2, yet they rapidly lost much of this ability due to the formation of an oxide layer on the surface, as determined by X-ray photoelectron spectroscopy, which likely prevented further photosensitization events. Addition of natural organic matter (NOM) did not significantly impact pSi's photosensitization abilities. The pSi lacked any intrinsic bactericidal properties on Escherichia coli and did not produce enough (1)O2 to considerably affect populations of a model virus, PR772, highlighting its relatively benign nature toward microbial communities. Results from this study suggest that the photoactivity of pSi is unlikely to persist in aqueous systems and that it may instead behave more similarly to silica particles from an environmental perspective.