Two isomers of N2O3, asym-N2O3 and sym-N2O3, are identified with infared (IR) absorption when mixtures of NO-NO2-Ar or NO-O-2-Ar are deposited onto a target at 13 K. Upon irradiation with a XeCl excimer laser at 308 nm, asym-N2O3 is converted to sym-N2O3 and a new form of N2O3, trans-cis N2O3; the latter is readily converted to sym-N2O3 upon further irradiation with red light. Assignments of IR absorption lines to each conformer in its isotopic variants are based on O-18-isotopic substitution and photoconversion experiments. For asym-N2O3, we observed O-18-isotopic shifts of a few vibrational modes previously unresolved in a N-2 matrix, confirming that O-atoms in the NO2 moiety are inequivalent. For sym-N2O3, a more nearly complete set of isotopic shifts for absorption lines at 1688.6, 971.0, and 704.6 cm(-1) enables us to provide refined assignments. In addition, lines at 1722.5 and 1721.1 cm(-1) are assigned to symmetric stretching modes of two terminal N=O groups of (ONONO)-O-18-O-16-O-16 and (ONONO)-O-18-O-18-O-16; they gain IR activity because C-2V symmetry is broken. Trans-cis N2O3 With an asymmetric ONONO structure is identified with absorption lines at 1704.5, 1665.7, 877.8, and 243.0 cm(-1). Isotopic experiments indicate that this species contains two nearly isolated N=O groups. Spectral assignments are supported by theoretical calculations with density-functional theories (BLYP and B3LYP); previous assignments of low-energy vibrational modes of asym-N2O3 and sym-N2O3 are revised based on comparison with calculations. Photoconversion among these isomers is discussed.