The effects of Sr substitution on superconductivity, and more particularly the changes induced in the hole doping mechanism, were investigated in Hg 2 (Ba 1−y Sr y ) 2 YCu 2 O 8−δ by a 'bond valence sum' analysis, with Sr content going from y = 0.0 to 1.0. A comparison with CuBa 2 YCu 2 O 7−δ and Cu 2 Ba 2 YCu 2 O 8 systems suggests a possible explanation of the T c enhancement from 0 K for y = 0.0 to 42 K for y = 1.0. The charge distribution among atoms of the unit cell was determined from the refined structure, for y = 0.0-1.0. It shows a charge transfer to the superconducting CuO 2 plane via two doping channels π(1) and π(2), i.e. through O2 apical -Cu and Ba/Sr-O1 bonds respectively.