  We present a path-integral Monte Carlo study of dissociation in dense
hydrogen ($1.75 \leq r_s \leq 2.2$, with $r_s$ the Wigner sphere radius). As
temperature is lowered from $10^5$ to 5000 K, a molecular hydrogen gas forms
spontaneously from a neutral system of protons and electrons. At high density,
$r_s < 2.0$, thermally activated dissociation is accompanied by decreasing
pressure, signaling the presence of a first order transition and critical
point. The decrease in electron kinetic energy during dissociation is
responsible for the pressure decrease and transition. At lower density the
phase transition disappears.
