The magnetic solids Cu(terpy)Mo2O7 (terpy = terpyridine) and Cu(OH)(p-pyc)H2O (p-pyc = p-pyridinecarboxylate) have a spin gap and possess chains of Cu2+ ions in which two different Cu center dot center dot center dot Cu distances alternate. On the basis of their reported crystal structures, the spin-exchange interactions of these compounds were examined by performing spin dimer analysis to determine whether an antiferromagnetic dimer or an alternating antiferromagnetic chain model is appropriate for their magnetic properties. Our analysis shows that an antiferromagnetic dimer model is correct for both compounds because of the anisotropic overlap between the magnetic orbitals of their Cu2+ sites.