A computer-controlled high temperature Guinier diffractometer system for accurate determination of lattice thermal expansion is described. A critical test of the system using α-Al2O3 (0.3μ polishing alumina) showed close agreement with the single crystal expansion data of Wachtman et al. Lattice thermal expansion of cordierite doped with the following dopants: Ge+4, P+5, Zn+2, Li+1 and Ca+2 was investigated. Of these the Li+1 at the 5% level (5% of Si+4 replaced by Li+1 + Al+3) produced the largest decrease in mean lattice expansion.