The (13)C- and (1)H- NMR spectra of isopropyl N-dodecanoylsarcosinate (SLIP) were measured in CDCl(3), CD(3)OD and in sodium dodecanoylsarcosinate (Sar) aqueous solution. The existence of both cis and trans isomeric forms are observed for SLIP just as for Sar. The trans/cis ratio of SLIP in a dilute CDCl(3) solution is over 3.2, which is indicating that SLIP-trans conformation is more stable in an organic solvent. However, the population of SLIP-trans isomer decreases in aqueous solutions. This may be explained by the interaction of the ester group and N-acyl chain. Addition of SLIP to the water/Sar solution causes the conformational change of both SLIP and Sar. With increasing SLIP concentration, the population of SLIP-trans isomer increases and Sar-cis isomer, which is known to be preferred in the monomer state, also increases. This phenomenon is discussed in the context of the phase transition behavior of SLIP/Sar/water system. At high SLIP and Sar concentration, broadening of (1)H signal relative to 2-CH(2) of Sar occurs both for trans and cis forms, however, broadening of that of Sar N-CH(2) and N-CH(3) is observed only for trans. Analysis of conformation change by NMR was proved to be useful method for phase behavior analysis of an acylamino acid surfactant.