Bioavailability of organic contaminants in soil has recently been a focus of interest but a lack of knowledge of its behavior during contaminant transport through saturated porous media, such as aquifers, is observed. This research presents a method, based on a generally accepted definition of bioavailability, that quantifies mass and toxic bioavailabilities of organic contaminants during their transport in terms of bioavailability rates. Elutions of pentachlorophenol (PCP) were performed using inoculated laboratory columns at two different pore-water velocities and on two different media. Both PCP and the toxic response (Microtox) were monitored at the exit of the columns for quantification of the rates. Results show that the non-dimensional mass bioavailability rate decreased with increasing pore-water velocity (from 0.52 to 0.15) and with increasing retention (from 0.15 to 0.02). The non-dimensional toxic bioavailability rate decreased with increasing retention (from 0.59 to 0.22). Results indicate that this method shows a potential for quantifying contaminant bioavailability according to both outcomes: biodegradation and toxicity.