We report on a comparative study of S(T) for a series of transition-metal double-perovskites A(2)BB'O-6 (A-Ca, Sr, Ba, and B, B' = transition metal ions), some of them known to have half-metallic ground states. For Sr2BB'O-6 with BB' = CrMo, CrW, CrRe, FeMo, and FeRe (ferrimagnetic with high Curie temperatures), S(T) is metallic, for B' = Mo and W it is n-type and for B' = Re, p-type. For A(2)FeMoO(6) (A = Ca, Sr, Ba), the crystallographic differences (monoclinic, tetragonal and cubic space-groups, respectively) are accompanied by prominent differences in their (metallic) S(T). For the insulating Sr2MnReO6 and Ba2MnReO6, the onset of ferromagnetic order below T-c similar to 120 K is marked by a steep drop of S(T) accompanied by only a slight change in the slope of ln rho versus 1/T-1/2. Significant conclusions were drawn from the experimental results without the need for elaborate models. (c) 2006 Elsevier B.V. All rights reserved.