The infrared photodissociation spectra of [(CO2) n(H2O)m]- (n=1-4, m=1, 2) are measured in the 3000-3800 cm-1 range. The [(CO2) n(H2O)1]- spectra are characterized by a sharp band around 3570 cm-1 except for n=1; [(CO 2)1(H2O)1]- does not photodissociate in the spectral range studied. The [(CO2) n(H2O)2]- (n=1, 2) species have similar spectral features with a broadband at ≈3340 cm-1. A drastic change in the spectral features is observed for [(CO2) 3(H2O)2]-, where sharp bands appear at 3224, 3321, 3364, 3438, and 3572 cm-1. Ab initio calculations are performed at the MP2/6-311++G** level to provide structural information such as optimized structures, stabilization energies, and vibrational frequencies of the [(CO2)n(H 2O)m]- species. Comparison between the experimental and theoretical results reveals rather size- and composition-specific hydration manner in [(CO2)n(H 2O)m]-: (1) the incorporated H2O is bonded to either CO2- or C2O4- through two equivalent OH⋯O hydrogen bonds to form a ring structure in [(CO2)n(H2O)1] -; (2) two H2O molecules are independently bound to the O atoms of CO2- in [(CO2)n(H 2O)2]- (n=1, 2); (3) a cyclic structure composed of CO2- and two H2O molecules is formed in [(CO2)3(H2O)2] -.