The Cr(NH3)(5)(py)(3+) ion has been obtained by metathesis of Cr(NH3)(5)(Me(2)SO)(3+) in pyridine, isolated as the perchlorate salt, and characterized by absorption (lambda(max) at 467, 352, and 260 nm) and emission spectra (lambda(max) at 668 nm, tau = 2.0 mu s at 20 degrees C in water) and by the py aquation rate (k = 5 x 10(-4) s(-1) at 80 degrees C). Ligand-field (LF) band irradiation in acid aqueous solution (10(-2) M HClO4) induces photoaquation of py (Phi = 0.26) and NH3 (Phi = 0.16). HPLC indicates that the latter reaction gives rise to both cis- and trans-Cr(NH3)(4)(py)(H2O)3+, with the predominance of the cis isomer. This is the first Cr(NH3)(5)X(z+) species where Phi(x) > Phi(NH3) : the result is compared with the predictions of various photolysis models and is taken as chemical evidence for pi-acceptance by the py ligand. The photostereochemistry is also discussed. The phosphorescence is totally quenched by Cr(C2O4)(3)(3-)(k(q) = 2.7 x 10(9) M(-1) s(-1)), while the photoreactions are only in part. On 470-nm excitation, the Phi(py)/Phi(NH3) ratio is similar to 1 and similar to 2 for the unquenchable and the quenchable contributions, respectively. Such a difference, suggesting at least two reactive precursors, can be interpreted in terms of the photochemistry proceeding from either the lowest doublet and quartet excited states or, alternatively, from the (4)E and B-4(2) states. Irradiation of the very distinct absorption of coordinated pyridine results in both doublet-state emission and loss of py and NH3. Comparison of this photobehavior with the LF results gives an efficiency of 0.7 for conversion of the py-localized pi pi* states into the Cr-localized LF states, confirmed by the wavelength dependence of the relative emission yields. Some py release (Phi = 0.03) is concluded to originate in the pi pi* states.