Spinodal decomposition in a near-critical binary fluid is examined for experimental scenarios in which the liquid is quenched abruptly by changing the pressure and the subsequent phase separation occurs with no heat flow from the outside, i.e., adiabatically. Equations of motion for the system volume and effective temperature are derived. It is shown for this case that the nonequilibrium decomposition process is well approximated as one of constant entropy, i.e., as thermodynamically reversible. Quantitative comparison, with no adjustable parameters, is made with the experimental light scattering data of Bailey and Cannell [Phys. Rev. Lett. 70, 2110 (1993), 10.1103/PhysRevLett.70.2110]. It is found that including these adiabatic effects accounts for most of the discrepancies between these experiments and previous isothermal theory. The equilibrium static critical properties of the isothermal theory are also examined, this discussion serving to justify some approximations in the current theory.