In 1972 and 1973, the CRC-APRAC Program Group on Diesel Exhaust carried out a fourth program to evaluate techniques for measuring concentration of hydrocarbon in diesel exhaust. The first two programs were conducted in 1967 and 1968. In them, a single cylinder diesel engine was shipped among 13 laboratories and each laboratory measured hydrocarbon emissions by their own method. Agreement among laboratories (instruments) was poor in both programs. The third program was conducted in 1970 at one laboratory on one engine. This time, agreement among instruments was much improved from the earlier programs. The fourth program was conducted to confirm these later results. In it, a multi-cylinder diesel generating set was circulated among 15 participating laboratories, and each laboratory measured exhaust hydrocarbon by methods that complied with SAE Recommended Practice J215, “Continuous Hydrocarbon Analysis of Diesel Exhaust.” They also measured hydrocarbon concentrations of two bottled gases having unknown compositions. Analyses were fairly consistent within laboratories both on bottled gases and engine exhaust: the standard deviations were 3% and 10% of the grand averages, respectively. However, analyses differed substantially among laboratories both on bottled gases and engine exhaust; the standard deviations were about 10% and 22% of the grand averages, respectively. These results scatter more than desirable for engineering measurements, and they indicate that further improvement should be sought in techniques for analyzing hydrocarbons in diesel exhaust.