Waste expanded polystyrene (EPS) represents a source of valuable chemical products like styrene and other aromatics. The catalytic degradation was carried out in a batch reactor with a mixture of polystyrene (PS) and catalyst at 450 ℃ for 30 min in case of Mg and at 400 ℃ for 2 h both for MgO and MgCO3 catalysts. At optimum degradation conditions, EPS was degraded into 82.20±3.80 wt%, 91.60±0.20 wt% and 81.80±0.53 wt% liquid with Mg, MgO and MgCO3 catalysts, respectively. The liquid products obtained were separated into different fractions by fractional distillation. The liquid fractions obtained with three catalysts were compared, and characterized using GC-MS. Maximum conversion of EPS into styrene monomer (66.6 wt%) was achieved with Mg catalyst, and an increase in selectivity of compounds was also observed. The major fraction at 145 ℃ showed the properties of styrene monomer. The results showed that among the catalysts used, Mg was found to be the most effective catalyst for selective conversion into styrene monomer as value added product.