Following addition of either the D- or the L-isomers of alanine, glutamine or glutamic acid or D-glucose, the CO2 production from an arable and a forest soil was measured until the pulses of CO2 production associated with substrate addition subsided. The maximum rate of additional CO2 production from the D-glucose amended soils occurred within the first 48 h for both soils. The greatest rates of additional CO2 production from L-amino acid amended soils occurred within 108 h for the forest soil and 60 h for the arable soil. Following addition of D-amino acids to the forest soil, the maximum rate of additional CO2 production was less than that following addition of the corresponding L-amino acid addition. However, for this soil the pulse of additional CO2 production following D-amino acid amendment lasted longer and by the time it had subsided (360 h), the total additional CO2 production did not differ between isomeric forms of the same amino acid. Following D-amino acid addition to the arable soil there were delays of between about 24 and 48 h before the onset of rapid additional CO2 production and the CO2 pulse subsided relatively rapidly. The total additional CO2 produced from the arable soil was significantly less for the D-amino acid than for the corresponding L-amino acid treatments. Successive additions of D-glucose led to significant increases in the subsequent rates of additional CO2 production from the forest soil, but not from the arable soil. Each successive L-amino acid amendment led to increases in the rate of additional CO2 production from both soils, as did successive additions of the D-amino acids to the forest soil. However, successive additions of the D-amino acids to the arable soil did not lead to consistent responses in the additional rate of CO2 production.