Phase lag spectroscopic measurements are performed for HI and DI in the 5d(pi,delta) resonance region. Measurements of the phase lags for both molecules reveals a strong isotopic effect, especially pronounced in the ionization channel. The energy dependence of the phase lag provides clear examples of continuum and resonance coupling effects that were previously predicted theoretically but have not yet been observed. It is demonstrated that phase lag spectroscopy is a sensitive technique for studying properties of molecular continua, furnishing information that is not available from phase-insensitive observables.