Passive diffusion tubes (PDTs) are an inexpensive and simple method to monitor air pollutants. Numerous studies have investigated the performance of PDTs for NO2 but little attention has been paid to PDTs for NOx. The aim of this study was to evaluate the performance of NOx PDTs in three different urban environments. Duplicate NOx and NO2 PDTs were co-located with chemiluminescence analysers at kerbside, urban centre and background sites in the city of Glasgow for twelve 1-week exposures. PDT measurements generally showed good temporal correlations with NOx and NO2 determined by the continuous analysers. However detailed evaluation showed PDT measurements were variously influenced by factors causing bias, according to individual site characteristics: positive bias in both NOx and NO2 PDTs due to wind-associated shortening of diffusion path; positive bias in NO2 PDTs due to within-tube chemical reaction between NO and O3; and, where NO concentrations were high, negative bias in NOx PDTs assumed due to i