Steady-state fluorescence and absorption spectra have been obtained in the Qy spectral region (690–780nm and 600–750nm, respectively) for several subunit-deficient photosystem I mutants from the cyanobacterium Synechocystis sp. PCC 6803. The 77K fluorescence spectra of the wild-type and subunit-deficient mutant photosystem I particles are all very similar, peaking at ∼720nm with essentially the same excitation spectrum. Because emission from far-red chlorophylls absorbing near 708nm dominates low-temperature fluorescence in Synechocystis sp., these pigments are not coordinated to any the subunits PsaF, Psa I, PsaJ, PsaK, PsaL, or psaM. The room temperature (wild-type–mutant) absorption difference spectra for trimeric mutants lacking the PsaF/J, PsaK, and PsaM subunits suggest that these mutants are deficient in core antenna chlorophylls (Chls) absorbing near 685, 670, 675, and 700nm, respectively. The absorption difference spectrum for the PsaF/J/I/L-deficient photosystem I complexes at 5K reveals considerably more structure than the room-temperature spectrum. The integrated absorbance difference spectra (when normalized to the total PS I Qy spectral area) are comparable to the fractions of Chls bound by the respective (groups of) subunits, according to the 4-Å density map of PS I from Synechococcus elongatus. The spectrum of the monomeric PsaL-deficient mutant suggests that this subunit may bind pigments absorbing near 700nm.