Energy transfer from tris(4,7-diphenyl-1, 10-phenanthroline)ruthenium(II) perchlorate (denoted by Ru(II)) to tris(4,7-diphenyl-1, 10-phenanthroline)osmium(II) perchlorate (denoted by Os(II)) has been studied in cast and Langmuir-Blodgett (LB) films. The work is focused on the packing effects of chiral metal complexes on energy-transfer rates. Three kinds of films were prepared: a cast film of a 100:1 mixture of Lambda -Ru(II) and Lambda -Os(II) (or Delta -Os(II)) (sample 1), a Y-type LB film of a 100:1 mixture of Lambda -Ru(II) and Lambda -Os(EI) (or Delta -Os(II)) (sample 2), and an alternative Y-type LB film of Lambda -Ru(II) and Lambda -Os(II) (or Delta -Os(II)) monolayers (sample 3). In all of these samples, stearic acid (SA) is added at a molecular ratio of [SA]/[Ru(II)] = 3 to stabilize a monolayer state. From the analyses of the decay curves of excited Os(II) (denoted by Os(II)*), the apparent unimolecular energy-transfer rate constant from excited Ru(II) (denoted by Ru(II)*) to Os(II), k(2), is determined. As a result, k(2) is found to. be larger for Lambda -Ru(II)*/Delta -Os(II) than for Lambda -Ru(II)*/Lambda -Os(II) in samples 1 and 3, while k(2) is larger for Lambda -Ru(II)*/Lambda -Os(II) than Lambda -Ru(II)*/-Os(II) in sample 2. The results are rationalized in term of the assumption that closer accessibility is attained for the packing of the pseudo-racemic pair of Lambda -Ru(II)/Delta -Os(II) than for that of the pseudo-enantiomeric pair of Lambda -Ru(II)/Lambda -Os(II), when one complex is located above the other, while the situation is reversed when one molecule is located by the side of the other.