The presence of NO2 gas in the environment poses a threat to life, both directly and indirectly. It is, therefore, essential that the concentration level of this gas is closely monitored in areas such as cities where population density is high. Various techniques already exist for concentration measurements of NO2 gas in air and, of course, each method has advantages and disadvantages. This paper describes the construction and characterisation of a relatively inexpensive, ratiometric, optical multipass gas cell which has good sensitivity (3.25 × 10−6 ppm−1) and exhibits a linear response over a large dynamic range (1–12000) ppm. It does, however, exhibit poor repeatability due to drift and a limiting resolution of lppm. Nevertheless, with further development, it may form the basis of a portable air quality monitor.