Low-temperature EPR spectra of Co(en)(3)CuCl5 . H2O single crystals were studied at X-band. The crystals contain layers of antiferromagnetically coupled Cu2Cl84- dimers, with singlet-triplet splitting 2J(0) = -14.8 cm(-1), separated by diamagnetic Co(en)(3) molecules. Superexchange coupling between dimers located in neighboring layers, determined from the merging effect of EPR lines from inequivalent sites, is J(1) = 0.00712 cm(-1) at room temperature, decreases to 0.00325 cm(-1) below 100 K, and then is temperature independent. Below 15 K the fine structure lines from S = 1 are resolved with the following zero-field splitting parameters : D-z = -0.0200 cm(-1), D-y = 0.0147 cm(-1), and D-x = 0.0053 cm(-1). The appearance of the structure is a result of a decrease in the number of magnetic dimers due to a rapid thermal depopulation of the triplet state when kT < 2J(0). A detailed analysis of this process allowed us to determine intralayer exchange coupling between dimers which amounts to J(2) = 0.041 cm(-1) at 100 K and decreases on cooling following triplet density rho(T) = [1 + 1/3 exp(-2J(0)/kt)](-1)) behavior. Hyperfine splitting is not resolved down to 4.2 K, and the relatively narrow lines indicate a strong mobility of the still remaining triplet excitons. The line width decreases on cooling down to about 100 K, and then a strong broadening appears resulting in the fine structure splitting. Contributions from the narrowing effect, triplet state density, and temperature variations of the zero-field splitting have been separated in Delta B-rho rho(T). A decrease in the zero-field splitting occurs on heating and, in the temperature range 4.2-100 K, is phenomenologically described by the equation D(T) = D-0(1 - aT - bT(-2)).