The sensitivity enhancement of cw intracavity dye laser spectra of I2 and of water vapor has been measured for four dye laser tuning systems, two cavity lengths, and a wide range of pumping powers. The longitudinal mode structure of the dye laser has also been examined. The sensitivity enhancement increases with the bandpass of the dye laser tuning element and is independent of the cavity length. It is only weakly dependent on pumping power except near threshold, where it climbs steeply. The experimental results are compared with predictions of several theories which seek to explain the basis for the enhancement. None of the predictions are in quantitative agreement with our results; and, except for the prediction that enhancement increases near threshold, the theories also give generally poor qualitative agreement with experiment.