A Miniature Continuous Monitor (MCM) system is compared in the laboratory with the Marcali method, which uses the Beckman Model B Spectrophotometer, for accuracy, reproducibility, reliability and ease of use in detecting toluene diisocyanate (TDI) concentration levels within the NIOSH exposure limits. Poor results initially obtained with the MCM are considered likely to have been caused by large erratic variations in pump flow, non-uniformity of the paper tape, errors in reading the tapes, experimental errors in sampling and variability of the results obtained with the analytical method used to establish the TDI concentration. Later results show that the MCM system gives good personal sampling data compared to the Marcali method, with a major advantage of giving a graphical record of a worker's exposure profile over an entire eight hour period. It is also less awkward to use. Interference by primary aromatic amines is not evaluated.