The biofiltration of dimethyl sulphide (Me(2)S) and other volatile sulphur compounds results in the accumulation of the metabolite sulphuric acid in the carrier material. Regeneration of an acidified (pH 4.7), Hyphomicrobium-MS3-inoculated compost biofilter degrading Me(2)S was not possible by trickling tap water (days 0-28) or a KH2PO4/K2HPO4 buffer solution (1.26 g PO43- 1(-1), pH 7) (days 29-47) over the bioreactor at a superficial liquid flow rate of 34 lm(-2) day(-1). Since the protons produced displaced nutrient cations (Na+, K+, Ca2+, Mg2+, NH4+) from the cation-exchange sites on the compost material, 95% of the SO42- was leached as the corresponding sulphate salts and not as sulphuric acid. Concomitantly, the pH of the compost material decreased from 4.7 to 3.9 over the 47 days rinsing period. Moreover, the rinsing procedure resulted in the leaching of essential microbial nutrients from the compost material, such as NH4+ (22.3% washout over the 47-day rinsing period) and PO43- (39.3% washout over the 28-day tap-water rinsing period). However, mixing limestone powder into the Me(2)S-degrading compost biofilter was a successful approach to controlling the pH in the optimal range for the inoculum Hyphomicrobium MS3 (pH 6-7). A stoichiometric neutralisation reaction (molar ratio CaCO3/H2SO4 = 1.1) was observed between the CaCO3 added and the metabolite of the Me(2)S degradation, while high elimination capacities (above 100 g Me(2)S m(-3) day(-1)) were obtained over a prolonged (more than 100 days) period.