Results of finite-difference, time-dependent numerical studies of the near field of subsonic, reactive square jets are presented. The simulations model space/time-developing compressible (subsonic) jets, using speciesand temperature-dependent diffusive transport, and finite-rate chemistry appropriate for H 2 combustion. Comparative measurements of entrainment for square jets are obtained based on evaluations of streamwise mass-flux to obtain an assessment on how the jet development is affected by chemical exothermicity and density differences between the jet and the surroundings. Depending on initial conditions (i.e., on the chemical exothermicity level implied by the initial reactant concentration), chemical energy release and expansion effects can be significant in determining reduced entrainment and initial jet growth relative to corresponding nonreactive jets. The instantaneous product formation rates are closely correlated with the local entrainment rates controlled by the vorticity bearing fluid. Instantaneous entrainment rates--based on the rate of increase of mass flux of rotational fluid--are found to be significant in the regions of roll-up and initial self-deformation of vortex rings, and then farther downstream, in the vortex merging region, where fluid and momentum transport between the jet and its surroundings are considerably enhanced by the presence of hairpin vortices aligned with the corners. Analysis of the combustion dynamics in terms of scalar mixing fraction diagnostics previously used in laboratory reactive turbulent jet experiments, was shown to be also potentially useful in characterizing their transitional regime by bringing out the relation between product formation rates and underlying fluid dynamical events.