The phosphorus fraction made available to organisms by the action of alkaline phosphatase, here termed phosphatase hydrolyzable phosphorus (PHP), was studied in central New York State streams. Annual mean PHP concentrations were <2.0 μg P∙L−1 for each of the four streams studied and were low relative to concentrations for lentic systems from the literature. The PHP to soluble reactive phosphorus (SRP) ratio, a measure of the relative importance of PHP for aquatic organisms, was typically in the 0.09–1.00 range, indicating that PHP could supply a significant amount of P at certain locations. Laboratory sorption experiments with varying ionic strength solutions and with Ca2+ versus Na+ suggested that PHP was probably bound to sediments by a different mechanism than SRP, one probably mediated by the organic moiety. Fine-grained sediments higher in organic matter sorbed excess PHP from solution at a greater rate than coarse-grained sediments, but also released more into PHP-depleted solutions.