An expression for the peak-to-peak sub-Doppler optical phase shift of two counterpropagating modes of light, to which the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) dispersion signal is proportional, valid for arbitrary degree of saturation, is derived. For low degrees of saturation it agrees with the expression for weakly saturating $\left(ws\right)$ conditions, $\left[\right(1+S{)}^{-1∕2}-(1+2S{)}^{-1∕2}]{\alpha }_0∕2$ , where S is the degree of saturation and ${\alpha }_0$ is the unsaturated peak absorption. However, the new expression, which can be written as $0.45S(1+S{)}^{-1}{\alpha }_0∕2$ , does not predict a distinct maximum as the $ws$ expression does; instead it predicts an optical phase shift that increases monotonically with S and levels off to $0.45{\alpha }_0∕2$ for large S. This alters the optimum conditions for the sub-Doppler NICE-OHMS technique and improves its shot-noise-limited detectability. The new expression is based upon the same explicit assumptions as the $ws$ expression but not the Kramers–Kronig relations, which are not valid for nonlinear responses, and is supported by experimental results up to $S=100$ . The new expression is expected to be valid for all techniques measuring sub-Doppler dispersion signals.