Steady state and time-resolved photoluminescence measurements of a homologous series of CdSe cluster molecules were performed over a broad temperature range (T = 5-200 K). The absorption and low temperature PLE onset of the clusters shifts systematically to the blue in smaller clusters, manifesting the quantum confinement effect. The emission in all cluster molecules is observed only at low temperatures and is red-shifted significantly from the absorption onset. It is assigned to optically forbidden transitions involving surface states, as substantiated by the ms range of lifetimes and by the involvement of low frequency vibrations of capping selenophenol ligands in the nonradiative relaxation of excited cluster molecules.1. Introduction Semiconductor clusters, composed of less than 100 atoms and still possessing the bonding type of the bulk semiconductor are interesting molecular models of the solid state [1-3]. Such compounds, usually referred to as cluster molecules, can be synthesized nowadays in preparative amounts and their structure and composition are determined precisely from single-crystal X-ray diffraction measurements (XRD) (Fig. 1a).We have investigated the optical properties of the cluster molecules, to determine if aside from serving as a structural model for the bulk semiconductor, they also display the zero-dimensional limit of its electronic properties.Size dependence of electronic structure and optical properties of larger semiconductor quantum dots revealing quantum confinement effects, have been studied extensively [4,5]. The studies of semiconductor cluster molecules, however, have been mostly limited to single compounds, thus restricting the extraction of size dependent features. Recent progress in the synthesis of CdSe cluster molecules has allowed us to perform a comprehensive size dependent spectroscopic study of a homologous series of compounds with 4, 8, 10, 17 and 32 Cd atoms [3,6,7], with the largest cluster molecule in the series already overlapping in size with the smallest CdSe nanocrystals. Particular attention was given to the nature of the emission and here we present the results of continuous wave (CW) and time-resolved (TR) wavelength and temperature dependent measurements of CdSe cluster-molecule fluorescence.