Rumen fermentation kinetics of oil palm fronds (OPF) supplemented or not with enzymes (Hemicell® or Allzyme SSF®) or yeasts (Levucell®SC or Yea-Sacc®) were studied through an in vitro gas production test (96 h) (exp. 1). In exp. 2, enzymes were supplemented to OPF pre-treated during 3 or 9 wk with either one of five white rot fungi strains. Yeasts and enzymes were tested both in active and inactive forms, which revealed the most appropriate set-up to distinguish between the rate of supplements as direct contributors to the fermentation substrate vs. stimulators of the fermentation of the basal substrate. In exp 1, addition of active and inactive Yea-Sacc® increased the apparently rumen degradable carbohydrates (ARDC) by 11%, whereas enzymes did not affect rumen degradability of non-inoculated OPF. Neither yeast nor enzymes influenced the rate of gas production of non-inoculated OPF, except for active Hemicell® at the low dose. In exp. 2, inoculation of OPF with Ceriporiopsis subvermispora for 3 wk and Lentinula edodes for 9 wk increased ARDC, but additional enzyme supplementation did not further improve ARDC or the rate of gas production.