Magnetic circular dichroism, magnetic circularly polarized luminescence, emission, and absorption spectra have been measured over the range 12 500-50 000 cm(-1) for zinc octaethylporphyrin isolated in an argon matrix at similar to 6 K. The spectra arise from transitions at a single matrix site. Values of g parallel to for the Q, B, and N states are 7.0 +/- 0.7, 1.0 +/- 0.3, and 1.2 +/- 0.9, respectively. The degeneracy of the Q state ((1)E(u) in D-4h symmetry) is lowered by Jahn-Teller and crystal-field effects, with the splitting of the Q(O,O) band being similar to 30 cm(-1) Analysis of the spectra in the Q-band region yields Jahn-Teller parameters v(JT) = 140 +/- 4 cm(-1) lambda(JT) = 1.27 +/- 0.5, and E(JT) = 26 +/- 3 cm(-1) and suggests that the crystal-field distortion is of the same symmetry as the Jahn-Teller effect and of magnitude 30 +/- 4 cm(-1). Qualitative indications are that Jahn-Teller effects in the B and P (lowest triplet) states are weaker than that in the Q state.