The stability of chlorophyll a, Chl a, in neat and mixed lipid monolayers at the nitrogen-water interface has been measured using Langmuir trough and HPLC techniques. In neat monolayers, at subphase pH less than 8, Chl a degrades rapidly to produce pheophytin a, Phe a. Above this pH, Chl a appears to hydrolyze, giving a new product, Chl a (hyd), spectroscopically similar to the parent compound. This latter process appears to involve alteration at the pentanone ring peripheral to the porphyrin structure. For mixed monolayer studies, four oleoyl-bearing lipids with headgroups differing in charge have been used to explore the influence of such headgroups on these two pH dependent processes. Consistent with a mechanism by which a negatively charged headgroup may enhance H3O+ concentration in the monolayer region, it has been shown that L-alpha-dioleoylphosphatidyl-DL-glycerol markedly increases pheophytinization while eliminating the formation of Chl a (hyd). By contrast, positively charged dioleoyl-N-(3-trimethylammoniopropyl)carbamoylglycerol serves to generate formation of Chl a (hyd) with no production of Phe a. In neutral but polar 1,2-dioleoyl-sn-glycerol monolayers, product yields were observed that are consistent with the larger acidity in the region of Chl a. Finally, L-alpha-dioleoylphosphatidylcholine, which bears a zwitterionic headgroup, significantly enhanced apparent Chl a hydrolysis. These results are interpreted in terms of alterations by the host lipid in pH local to Chl a.