This study provides insight into the mechanisms that govern morphology in microparticles processed using precipitation by a compressed antisolvent. We explore the time scale of surface tension evolution in jets of miscible fluids injected into critical and near-critical solvents to determine whether the jets atomize into droplets or simply evolve as gaseous plumes. Classical jet breakup length equations, modified with time-dependent surface tension, accurately predict observed breakup lengths over a range of liquid miscibilities. Linear jet breakup theory can be applied successfully to near critical conditions. The aerodynamic reduction factor remains constant over a wide range of pressures. However, under miscible conditions, calculations show that surface tension in a 10-cm/s round jet of methylene chloride in carbon dioxide at 8.5 MPa and 35 degrees C approaches 0.01 mN/m in I mu m. Because this distance is shorter than characteristic breakup lengths, distinct droplets never form. Rather, the jets spread in a fashion characteristic of gaseous jets, whose mixing is well described by the gaseous fluid mixing theory. Presumably, microparticle formation results from gas phase nucleation and growth within the expanding plume, rather than nucleation within discrete Liquid droplets.