AbstractA method has been developed to determine α, β and γ diastereoisomers of hexabromocyclododecane (HBCD), a brominated flame retardant, in sewage sludge, based on the ultrasonic‐assisted extraction (UAE) of samples with dichloromethane–ACN (1:1) and the subsequent clean‐up of extracts by dispersive solid phase extraction with primary–secondary amine. Levels of HBCD diastereoisomers were determined by LC coupled with ESI MS/MS. Evaluation of the matrix effect showed a high ion suppression for all the diastereoisomers studied, which was counteracted by using a mixture of labelled HBCD diastereoisomers as internal standards. This method yielded recoveries in the range of 79.6–112.5% with SDs equal or lower than 9.1 The limits of detection were 0.3 ng/g for α‐ and β‐HBCD and 0.2 ng/g for γ‐HBCD. The developed method was successfully applied to 19 sludge samples collected from the province of Madrid (Spain). In most of the samples, β‐HBCD was below the method detection limit, whereas α‐ and γ‐HBCD were quantified in all of the sludge samples, and γ‐HBCD was the predominant diastereoisomer in 63% of the analyzed samples and α‐HBCD predominated in the rest.