Integration of relatively simple logic gates into high-level circuits (e.g., half-adder) is highly important not only for the potential application in molecular level devices but also for the understanding of complex mechanisms of some important biological processes. In this report, we demonstrate a novel kind of theoretical "monomolecular" half-adder based on multistate/multifunctional photochromic spiropyrans for the first time. Spiropyran molecule (SP1) can be transformed to MCl, (MClFe3+)-Fe-. and SP1(+.) under the appropriate actions of 11 (UV light) and 12 (ferric ion). The intrinsic network of the transformations among the four states (SPl, MCl, (MClFe3+)-Fe-., and SP1(+.)) reveals the distinct and interesting properties. The behaviors of the absorption changes at 420 and 520 nm correspond to those of an "AND" gate and an "XOR" gate, respectively. Since the absorption changes at both 420 and 520 nm are based on the entity of a single molecule of spiropyran (e.g., SPl) and under the same external inputs, the AND and XOR gates can be "operated" in parallel. Therefore, the absorption spectral variations of the solution of spiropyran in response to UV light irradiation and ferric ion mimic the function of a "half-adder" that can perform simple arithmetic addition.