Acetic acid is a well-studied system in both the gas phase and in inert gas matrices [1]. However, there are still unidentified characteristic dimer bands, which are important for studying molecular clustering. Here, we show that MI-FTIR is a useful tool for investigating different dimer states and accurately assigning both monomer and dimer bands by combining matrix annealing with quantum chemical calculations. Annealing was shown to be effective in forming both dimers of acetic acid and in increasing the dimer to monomer ratio. Throughout the study, the monomer bands were assigned using VCI calculations, which were previously shown to be highly accurate [2]. Dimer bands were assigned using calculations based on the harmonic approximation. The computationally cheaper harmonic calculations were shown to be sufficient in assigning dimer bands accurately when used in combination with annealing. This work also compares the matrix gases argon and neon. A difference in matrix shift between argon and neon, compared to the gas phase was observed, and matrix splitting in neon was also observed. Our results can be applied to build a framework for characterising monomer and dimer bands of bigger, more complicated molecules like pinonic acid, pinic acid and other organic or inorganic analytes.