This analytical method is a validated measurement procedure for the determination of isopropylated triphenyl phosphate in workplace air in a concentration range of one tenth up to twice the currently valid OEL or MAK value of 1 mg/m³ I. Isopropylated triphenyl phosphate is a complex isomeric mixture of variable composition. Sampling is performed by drawing a defined volume of air through a quartz filter spiked with triphenyl phosphate-d₁₅ (ISTD). The flow rate is set to 3.5 ml/min and sampling period can be 2 h up to 8 h, which correspond to a sampling volume of 420 to 1680 l. The collected isopropylated phenyl phosphates are extracted with ethyl acetate in an ultrasonic bath and a heated horizontal shaker. For purification, a solid phase extraction can be used. Analysis is performed by means of gas chromatography using an mass selective detector. The quantitative determination is based on a calibration function, whereby the quotients of the peak area of the isopropylated phenyl phosphates to triphenyl phosphates-d15 are plotted against the peak areas of the organic phosphorus compounds. The relative limit of quantification (LOQ) is 0.050 mg/m³ based on an air sample volume of approx. 420 l. The mean recovery of this method is 98,9% ± 6% and the expanded uncertainty for the overall measurement method 30.7%.