Core-shell polyurea microcapsules with a 40% fragrance load were prepared by interfacial polymerization of guanidine and a technical polyisocyanate pre-polymer containing mainly the biuret trimer derived from hexamethylene di-isocyanate (HDI). Residual free polyisocyanates, were still present at a level slightly above 100 mg NCO functional group per kg as determined by liquid chromatography hyphenated with tandem mass spectrometry of HDI and of its biuret trimer. This level was decreased by a factor of about 10 when the polymerization process was allowed to proceed for a longer time and by a factor of about 500 when guanidine or NaOH were added to the microcapsule suspension to act as scavengers. In these cases, polyisocyanate conversion was observed to proceed for about one month when the microcapsules were stored at room temperature before reaching a plateau at a level below 1 mg NCO/kg. Overall, ammonia was the most efficient polyisocyanate scavenger as no residual HDI biuret trimer and only less than 2 mu g NCO/kg as HDI were detected at the end of the process, a level which had dropped below the limit of detection of 0.25 mu g NCO/kg after about 40 days of aging at room temperature. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 3074-3080,2009