The increasing use of polystyrene foam leads to serious impact in the waste collection and disposal. Recycling of these materials is difficult due to high transportation costs. Therefore, the dissolution in a suitable solvent is one of the most efficient and cheapest processes to recycle PS foam. In this work, viscosity of polystyrene semi-rigid foam dissolved in "green" solvents was measured and Huggins constant and critical concentration regions were determined. The solvents used were two monoterpenes, d-limonene and p-cymene. The viscosity of extremely dilute solutions was measured with an Ubbelohde type capillary viscometer, and for other solutions a Krebs type digital viscometer was used. The Huggins constant value was determined as 0.57 for d-limonene and 0.88 for p-cymene. The critical concentration that marks the transition from the extremely dilute to dilute regions was determined as 0.15 g/mL for d-limonene and 0.12 g/mL for p-cymene, and the critical concentration that marks the transition from the dilute region to the region that coils overlap and entanglements are formed was determined as 0.35 g/mL for both solvents. These measurements are important for the evaluation and development of dissolution recycling process.