The title compound Rb-14(Mg1-xInx)(30) (x = 0.79-0.88) has been obtained from high-temperature reactions of the elements in welded Ta tubes. There is no analogous binary compound without Mg. The crystal structure established by single-crystal X-ray diffraction means (space group P (6) over bar 2m (No. 189), Z = 1 and a = b = 10.1593(3) angstrom, c = 17.783(1) angstrom for x = 0.851) features two distinct types of anionic layers: isolated pentacapped trigonal prismatic In-11(7-) clusters and condensed [(MgxIn1-x)(5)In-14](7-) layers. The latter consists of analogous M-11 (M = Mg/In) fragments that share prismatic edges and are interbridged by trigonal M-3 units. The structure shows substantial differences from related A(15)TI(27) (A = Rb, Cs) in which the cation A that centers a six-membered ring of TI11 fragments is replaced by M-3. Both linear muffin-tin orbital and extended Huckel calculations are used to analyze the observed phase width and site preferences. We further utilize the results to rationalize the distortion of the M-11 fragment in the condensed layer and also to correlate with electrical properties. An isomorphous phase region (RbyK1-y)(14)(Mg1-xInx)(30) (y = 0.52, 0.66 for x = 0.79) is also formed.