The K- and Na-synthetic analogues of the fumarolic mineral ilinskite have been synthesized by the chemical vapor transport (CVT) reactions method. The A[Cu5O2](SeO3)(2)Cl-3 (A (+) = K+, Na+) compounds crystallize in the orthorhombic space group Pnma: a = 18.1691(6) , b = 6.4483(2) , c = 10.5684(4) , V = 1238.19(7) (3), R (1) = 0.018 for 1957 unique reflections with F > 4 sigma (F) for K[Cu5O2](SeO3)(2)Cl-3 (KI), and a = 17.7489(18) , b = 6.4412(6) , c = 10.4880(12) , V = 1199.0(2) (3), R (1) = 0.049 for 1300 unique reflections with F > 4 sigma (F) for Na[Cu5O2](SeO3)(2)Cl-3 (NaI). The crystal structures of KI and NaI are based upon the [O2Cu5](6+) sheets consisting of corner-sharing (OCu4)(6+) tetrahedra. The Na-for-K substitution results in the significant expansion of the interlayer space and changes in local coordination of some of the Cu2+ cations. The A (+) cation coordination changes from fivefold (for Na+) to ninefold (for K+). The CVT reactions method provides a unique opportunity to model physicochem