The semiarid and arid zones cover a quarter of the global land area and support one-fifth of the world's human population. A significant fraction of the global soil–atmosphere exchange for climatically active gases occurs in semiarid and arid zones yet little is known about these exchanges. A study was made of the soil–atmosphere exchange of CH4, CO, N2O and NOx in the semiarid Mallee system, in north-western Victoria, Australia, at two sites: one pristine mallee and the other cleared for approximately 65 years for farming (currently wheat). The mean (± standard error) rates of CH4 exchange were uptakes of −3.0 ± 0.5 ng(C) m−2 s−1 for the Mallee and −6.0 ± 0.3 ng(C) m−2 s−1 for the Wheat. Converting mallee forest to wheat crop increases CH4 uptake significantly. CH4 emissions were observed in the Mallee in summer and were hypothesized to arise from termite activity. We find no evidence that in situ growing wheat plants emit CH4, contrary to a recent report. The average CO emissions of 10.1 ± 1.8 ng(C) m−2 s−1 in the Mallee and 12.6 ± 2.0 ng(C) m−2 s−1 in the Wheat. The average N2O emissions were 0.5 ± 0.1 ng(N) m−2 s−1 from the pristine Mallee and 1.4 ± 0.3 ng(N) m−2 s−1 from the Wheat. The experimental results show that the processes controlling these exchanges are different to those in temperate systems and are poorly understood.