The mass analyzed threshold ionization (MATI) spectrum of CD3, recorded using two-colour (2 + 1') multiphoton excitation via the Q-branch transition to the 3p, Rydberg state, is reported. The fast CD3 radicals are produced by photodissociation of CD3I at 280 nm. The spectroscopy provides a possible route to state-selection of rovibronic levels of the CD3- ion, or for Rydberg tagging of the CD3 fragments in a photodissociation experiment. The MATI spectrum is simulated using a simple Hund's case (b) to Hund's case (d) intensity model. The variable parameters optimized in the model are the ratio of radial transition moments to the s and d channels (1:1), the MATI bandwidth (similar to 15 cm(-1)) and asymmetric lineshape (determined by the discrimination field and the high-n Rydberg decay), and the ionization potential of CD3 (79287 +/- 5 cm(-1)).