Using a simple criterion for the deflection of a constant-viscosity upwelling or downwelling by an endothermic phase transition, the scaling of the critical phase buoyancy parameter Pcrit with the important lengthscales is obtained. The derived trends match those previously observed in time-dependent numerical simulations, implying that geometry is the dominant factor in determining the propensity to layering. For a sinusoidal temperature anomaly, Pcrit is found to be proportional to wavelength, so that a stronger phase change is required to stop longer wavelengths, in accord with observations from three-dimensional numerical simulations. For more realistic Gaussian upwelling and downwelling features, the dependence of Pcrit on the width of feature, spacing of features, depth of phase transition and width of phase transition are determined for idealized internally heated and basally heated systems. Narrow upwellings and downwellings are deflected more easily than broad ones, providing a first-order explanation for the increased propensity to layering as Rayleigh number is increased. Internal heating is found to strongly favor deflection, particularly when the phase change is at shallow depth. For basally heated systems, the depth of the phase transition is found to be relatively unimportant in determining the value of Pcrit for which both upwellings and downwellings are deflected. In contrast, for internally heated systems, a shallower phase transition strongly favors layering. Only weak dependence of Pcrit on the spacing of upwellings and downwellings is found. A narrower phase transition enhances deflection. ¿ American Geophysical Union 1995