The partial high-resolution phase diagram of the ammonium pentadecafluorooctanoate (APFO)/H2O system (weight fraction of APFO from 0.350 to 0.630) has been established using N-14 NMR to determine the liquid crystalline phase transition temperatures. Comparison between the APFO/H2O phase diagram and the previously published APFO/D2O one reveals that at comparable volume fractions the phase transition temperatures are lower in the APFO/H2O system, but the difference between these decreases with increasing volume fraction of surfactant (increasing temperature). This isotope effect on the phase behavior can be understood in terms of phase transitions which are driven by hard particle interactions together with changes in the micellar self-assembly attributed to a tighter binding of ammonium ions to surface carboxylate groups in the case of D2O, leading to larger micelles in D2O than in H2O at corresponding volume fractions and temperatures. The magnitude of the solvent isotope effect is similar to that previously determined in the closely related cesium pentadecafluorooctanoate (CsPFO)/water systems, an observation which is consistent with the presence of a counterion-surface interaction via bridging water molecules. Polynomials are presented, which give the liquid crystalline phase transition temperatures (to an accuracy of +/-0.3 K) as a function of weight fraction of surfactant, for both the APFO/H2O and APFO/D2O systems.