We report on the first results with resonance enhanced two-photon spectroscopy of magnetically trapped atomic hydrogen, exciting the transition from the
1S ground state to the
3S or the
3D state. The spectroscopic method introduces tunable transparency to resonant optical studies of cold dense gases. It combines sensitivity (due to the near resonant
2P state) with high resolution (in principle limited only by the
3S state lifetime), making it a precision tool for optical investigation of ultracold H gas. To demonstrate its potential, the dynamic evolution of H gas during evaporative cooling is quantitatively investigated by analysis of two-photon absorption spectra.