A system for detecting if a refrigerant in an HVACR system is contaminated. The system includes a detector that detectes a property of a refrigerant gas in a vapor space and compares the detected property with a reference property for uncontaminated refrigerant gas. The detector determines that a contaminant is present in the refrigerant gas if the detected property is different from the reference property. The property detected by the dectector can be either a speed of sound through the refrigerant gas or a thermal conductivity of the refrigerant gas. Also disclosed is a method for detecting contamination of refrigerant gas in a HVACR system that includes detecting, by a detector, a property of refrigerant gas in a vapor space of the HVACR system.