In-situ Cd K-edge EXAFS analysis of colloidal CdS nanocrystallites (CdS-DMF, mean diameter 3.5 nm, hexagonal) prepared by reacting Cd2+ and H2S in N,N-dimethylformamide (DMF) was performed to clarify their microscopic surface structure in solution. The structure of the [Cd-4(SC6H5)(10)](2-) cluster (Cd-4-cluster) in DMF was also examined. EXAFS analysis revealed that CdS-DMF is stabilized by solvation of the oxygen atoms of DMF to the cadmium atoms on the surface of CdS nanocrystallites as well as to Cd-4-clusters in the solvent. The solvated surface structure of CdS nanocrystallites was affected by the addition of pentafluorothiophenol or H2S to the system. The correlation between the microscopic surface structure and optical properties of CdS nanocrystallites is discussed in terms of a model in which photogenerated carriers are trapped at lattice imperfections on the surface, i.e., surface vacancies.