We analyze the second order single particle longitudinal dynamics in a quasi-isochronous storage ring. We expand the momentum compaction factor to include the effects of second order terms taking sextupoles into account and of transverse betatron oscillations. The introduction of nonlinearities due to higher order terms results in a second stability region for longitudinal phase oscillations, in addition to the well known linear stable operation point. The conditions for this new solution to fall within the energy acceptance of the storage ring are presented. Inclusion of transverse motion coupling may lead to either a reduction or an enhancement of the stable longitudinal phase-space regions. The analysis is applied to the LNLS 1.15 GeV UVX electron storage ring, indicating that it should be possible to operate this ring in a quasi-isochronous mode. © 1993.32901/02/15915Deacon, (1981) Phys. Rep., 76, p. 5Proc. of the Workshop on 4th Generation Light Sources (1992) SSRL 92/02Pellegrini, Robin, (1991) Nucl. Instr. and Meth., 301 A, p. 27Hama, Takano, Isoyama, Proc. of the Workshop on 4th Generation Light Sources (1992) SSRL 92/02, p. 208Lin, Jahnel, Tavares, (1992) Proc. of the 3rd European Particle Accelerator Conf., p. 450. , See, for instanceSands, The Physics of Electron Storage Rings (1979) An Introduction, , See, for instance, SLAC 121Wiedemann, Accelerator Physics-Lecture Notes (1991) ICTP School on Use of Synchrotron Radiation in Science and Technology, , Trieste, ItalyDelahaye, Jäger, (1986) Particle Accelerators, 18, p. 183Pellegrini, Robin, (1991) IEEE Particle Accelerator Conf., p. 398. , San FranciscoWiedemann, (1992) SSRL ACD Note 125