Continuous measurement of H2S concentration around a heavy water plant has been done during summer period using indigenously developed H2S monitor. Diurnal spectrum of hourly mean and maximum H2S levels has been studied and data have been divided into 2 periods of the day. Hourly mean data have been analysed statistically with log normal distribution and hourly maximum data with extreme value distribution. The modal value of hourly maxima for summer period, which works out to be 387 ppb, is much below the TLV of H2S for occupational exposure.