Thermal excitation and zero-point motion bring about fluctuations in the gauge-invariant phase coupling across Josephson junctions. These fluctuations are studied in a simple picture incorporating both the resistively shunted junction (RSJ) model and the Ambegaokar-Baratoff theory. A diagram of the parameter space of a junction delineating a boundary between the phase-coupled and phase-decoupled regimes is obtained. Estimates of the root-mean-square voltage fluctuations justify the use of the normal-state tunneling resistance rather than the temperature-dependent quasiparticle tunneling resistance in the RSJ model at the boundary. © 1986 The American Physical Society.