The valence transition in Ce1−xThx is isomorphic, and, hence, analogous to a liquid‐vapor transition (the order parameter being the volume V); and it exhibits mean‐field behavior. We have found that in the critical region the x−T phase boundary is nearly parallel to the x‐axis, and that the data for resistivity R (which couples linearly to the order parameter) fits a Landau equation of state of the approximate form 0.35(ΔR/R0)3+0.67 (Δx/x0)(ΔR/R0) =ΔT/T0−0.07 Δx/x0 This is in analogy to a mean‐field ferromagnet with {M,H,T}∠{V,T,x}; consequently dR/dT‖max∠ (Δx)γ with γ=1, as demonstrated by the data. The mean field nature is due to the electroelastic coupling associated with the large volume change. This coupling enters the free energy as the Δx(ΔV)2 term, which may reflect the dominance of anharmonic strain with increased alloying. Magnetic susceptibility and specific heat measurements reveal that both the average magnetic moment and entropy couple linearly to the order parameter. The latter finding is consistent with the existence of the term −ΔVΔT in the free energy as implied by the above equation of state.