A procedure was developed for the estimation of cyanuric-acid (108805) (CA) or trichloroisocyanuric-acid (87901) (TICA) in air samples. The method involved air sampling with a 37 millimeter polyvinyl-chloride (PVC) membrane filter, recovery using a phosphate buffer, and measurement using high performance liquid chromatography with ultraviolet detection. Air sampling was conducted at a rate of 1.6 liters per minute for 6.6 hours at a manufacturing facility at which TICA was being produced from CA by a chlorination process. Air samples were collected in a part of the facility designated for CA purification so that the presence of TICA in this area was considered negligible. Results showed that the average recoveries from PVC filters were 0.98 to 1.00 after fortification of filters with 12 to 412 micrograms (microg) of CA. The limit of detection (LOD) and lower limit of quantitation (LOQ) of CA were 0.3 to 0.75 microgram (microg) and 1.0 to 2.5microg per sample, respectively. Average recoveries of TICA were 0.83 and 0.98, after fortification of glass surfaces with 12 to 424microg TICA. The LOQ for TICA was about 6microg per sample, and its LOD in solution was 0.54microg. Results from analyses of blind spiked samples of both CA and TICA corroborated the validity of the field samples. TICA underwent deterioration on standing. The authors conclude that the method is suitable for the measurement of TICA in view of its convenience for routine analysis. TICA in air samples should be measured within 1 week after collection.