Beer’s law plots for the ore-bridged d(1)-d(1) dimers [Mo2O3{S2P(OR)(2)}(4)] (R = Et, 3; R = Ph, 4; R = Me, 5) exhibit positive deviations from linearity, suggestive of dissociative equilibria. In the case of 3 it has been established by P-31 NMR that this is a consequence of the disproportionation equilibrium between 3 and its d(2) and d(0) disproportionation products [MoO{S2P(OEt)(2)}(2)] (6) and [MoO2{S2P(OEt)(2)}(2)] (7), both of which have been independently prepared. P-31 NMR spectroscopy has been used to measure the dissociation constants, K-diss, for 3, 4, and 5 at various temperatures, and hence to determine Delta G(298), Delta H and Delta S values of 4.43 +/- 0.05 kcal mol(-1), 12.1 +/- 1.0 kcal mol(-1), and 25.6 +/- 4.2 cal mol(-1) K-1 for 3; 4.08 +/- 0.06 kcal mol(-1), 11.2 +/- 1.3 kcal mol(-1), and 23.8 +/- 4.2 cal mol(-1) K-1 for 4; and 3.68 +/- 0.06 kcal mol(-1), 12.6 +/- 1.0 kcal mol(-1), and 30.0 +/- 4.5 cal mol(-1) K-1 for 5. A single-crystal diffraction study has established that 5 (monoclinic space group P2(1)/c; with a = 8.235(3) Angstrom, b = 16.544(5) Angstrom, c = 11.444(5) Angstrom, beta = 103.79(2)degrees, Z = 2, and R = 4.7%) has a typical [(Mo2O3)-O-V](4+) core with a linear bridging ore group with an anti orientation for the terminal oxo groups. Solutions of 3, 4, and 5 are irreversibly bleached by UV light (<400 nm), but are unaffected by visible light (>400 nm) despite the presence of intense absorptions at 500 nm (epsilon(500) = 11 800, 27 200, and 16 600 L mol(-1) cm(-1) respectively for 3, 4, and 5). Disappearance quantum yields for photolysis of 3, 4, and 5 at various irradiation wavelengths are modest in the UV (Phi = 10(-3)-10(-2)) but negligible at 510 nm (Phi(510) < 10(-4)), giving rise to Phi(310):Phi(510) ratios of >150, >180, and >60 respectively for 3, 4, and 5. It is proposed that this marked wavelength dependence arises because the d(1)-d(1) dimers are not themselves photoactive, but are destroyed through photolysis of the d(0) disproportionation products [MoO2{S2P(OR)(2)}(2)] as confirmed for the case of 3 by independent determination of the disappearance quantum yield for 7 as 0.12 at 310 nm.