The cyclic (C-n) light harvesting 2 (LH2 or B800-850) complexes of Rps. acidophila (strain 10 050) and Rb. sphaeroides, isolated under identical conditions, are compared using the title spectroscopies. Thermal broadening and shifting data for the B850 absorption band reveal a structural change near 150 K for both species in the glycerol:water solvent used. The linear regions of thermal broadening above and below this temperature are shown to be consistent with dephasing via phonon-assisted relaxation between the B850 ring’s exciton levels, which contribute to the B850 absorption band. The theoretical model used predicts, for both species, that the nearest neighbor coupling(s) between bacteriochlorophyll a (BChl a) molecules of the B850 ring is (are) significantly stronger, ca. 35%, for the low-temperature structures. Moreover, the linear thermal broadening rates of Rb. sphaeroides are significantly lower than those of Rps. acidophila for both the low-and high-temperature regions. Analysis of the difference in rates with the above model indicates that the nearest neighbor BChl a-BChl a coupling(s) is ca. 20% weaker for Rb. sphaeroides at all temperatures. The observation that the thermal shift rate for the B850 band of Rb. sphaeroides is 2.2 times smaller than that of Rps. acidophila is consistent with this weaker coupling. Pressure shift data for the B800 band indicate that the compressibility (kappa) for Rb. sphaeroides is significantly larger than for Rps. acidophila, suggesting that the weaker excitonic coupling between B850 molecules of Rb. sphaeroides stems, at least in part, from looser packing of its alpha,beta-polypeptide pairs. A higher kappa value for Rb. sphaeroides provides an explanation for the observation that the linear rates for pressure broadening and shifting of the B850 band for the two species are similar. Although the pressure-and temperature-dependent data for the B800 band of both species are consistent with weak excitonic coupling between nearest neighbor B800 molecules, the data for the B850 band (including pressure shifting of zero-phonon holes burned into the lowest exciton level of the B850 ring (B870)) require interpretation in terms of strong coupling. Although large, the pressure-shift rate for B870 holes burned on the high-energy side of the B870 band (-0.46 cm(-1)/MPa) is a factor of 1.3 lower than on the low-energy side. An interpretation for this variation in terms of energy disorder is given. Zero-phonon hole action spectra (4.2 K) for the B870 exciton level are presented that yield similar inhomogeneous widths for the B870 band of both species, similar to 120 cm(-1). For both species the apparent displacement of this band below the maximum of the B850 band is close to 200 cm(-1). A theoretical discussion of the relationship between the apparent displacement and excitonic level structure in the absence of energy disorder is given in the accompanying paper.