The environmental occurrence of perfluorooctane sulfonate (PFOS) can arise from its direct use as well as from transformation of precursors ((N-alkyl substituted) perfluorooctane sulfonamides; FOSAMs). Perfluorooctane sulfonamidoethanol-based phosphate (SAmPAP) esters are among numerous potential PFOS-precursors which have not been previously detected in the environment and for which little is known about their stability. Based on their high production volume during the 1970s-2002 and widespread use in food contact paper and packaging, SAmPAP esters may be potentially significant sources of PFOS. Here we report for the first time on the environmental occurrence of SAmPAP diester in marine sediments from an urbanized marine harbor in Vancouver, Canada. SAmPAP diester concentrations in sediment (40-200 pg/g dry weight) were similar to those of PFOS (71-180 pg/g). A significant (p < 0.05) correlation was observed between SAmPAP diester and N-ethyl perfluorooctane sulfonamido acetate (an anticipated degradation product of SAmPAP diester). ∑PFOS-precursor (FOSAM) concentrations in sediment (120-1100 pg/g) were 1.6-24 times greater than those of PFOS in sediment. Although SAmPAP diester was not detected in water, PFOS was observed at concentrations up to 710 pg/L. Among the per- and polyfluoroalkyl substances monitored in the present work, mean log-transformed sediment/water distribution coefficients ranged from 2.3 to 4.3 and increased with number of CF(2) units and N-alkyl substitution (in the case of FOSAMs). Overall, these results highlight the importance of FOSAMs as potentially significant sources of PFOS, in particular for urban marine environments.