Tunable diode laser absorption spectroscopy has applied to the gas phase hydrogen peroxide in ambient air with sub-ppbv (parts per billion by volume) detection limits for measurement times of the order of minutes. The methods for calibrating the instrument and for assuring the absence of spectroscopic and sampling interferences at the level of our present detection limits are described. Ambient air monitoring with our system indicates that the hydrogen peroxide mixing ratio is often <0.3 ppbv. Five-minute average mixing ratios of up to 2.9 ppbv have been measured at sites in southwestern Ontario, Canada, during the summer months of 1984 and 1985, while the highest 1-hour average value observed was 2.1 ppbv.