Monovinylidene aromatic polymers, e.g., general purpose polystyrene or high impact polystyrene, are plasticized with a nonfunctionalized, nonmineral oil, e.g., a vegetable oil such as canola oil, cottonseed oil or the like. The mixture of monovinylidene aromatic polymer and oil is typically and preferably free of any significant amount of functionalized nonmineral oil, and it typically contains at least 0.1 weight percent of the nonfunctionalized, nonmineral oil, alone or in combination with a mineral oil, based on the weight of the polymer. The nonmineral oil has an iodine value of less than 120, and a hydroxyl number of less than 20.