An in vitro study was conducted to investigate effects of inoculum source (i.e., sheep versus buffalo rumen fluid) on gas production profiles, and to evaluate the suitability of various mathematical equations to fit the profiles and provide accurate values of degradation attributes. Incubations were completed using ruminal fluid obtained from sheep or buffalos fed the same diet (600 g/kg grass hay and 400 g/kg concentrate). Kinetics of fermentation of five feeds commonly fed to ruminants, being maize silage, grass silage, wheat straw, barley grain and a mixed hay, were studied with a gas production technique using an automated pressure evaluation system (APES). Ruminal fermentation characteristics (i.e., substrate disappearance, pH and VFA production) were determined after 120 h of incubation. Five mathematical functions (i.e., exponential, France, Gompertz, logistic, Morgan) were fitted to the experimental data to estimate rate and extent of feed degradation. Model comparison was based on goodness-of-fit assessed from analysis of residual variance and Akaike's information criterion. The logistic and Morgan functions were best overall, although the goodness-of-fit attained with all models was considered acceptable. Except for the Morgan, there were small differences among models in values derived for extent of degradation. There were differences between sources of inoculum in gas production measured at intermediate times (i.e., gas volumes with buffalo rumen fluid of 133 and 164 ml/g organic matter (OM) at 24 h for grass hay and silage, respectively, were smaller than those with sheep rumen fluid of 182 and 208 ml/g OM), but not at earlier or later incubation times. As a result of this trend, shorter half times, faster fermentation rates and higher extents of degradation occurred when feeds were incubated in sheep (estimated OM degradability was 0.308 and 0.402 g/g OM for grass hay and silage, respectively) compared with buffalo rumen fluid (0.246 and 0.330 g/g OM). Differences were larger for more fibrous substrates (i.e., grass silage, straw and hay) and negligible for barley grain.