The aim of this study was to determine the effects of the addition of different amounts of fumaric acid (FA) and malic acid (MA) to the leaves of Robiniapseudoacacia (Black locust, acacia), Prunus laurocerasus(cherry laurel), Quercus cerris (oak), and Camellia sinensis (tea factory wastes, TFW), to improve their value as alternative feeds for ruminants. The parameters examined were the in vitro true digestibility of feed (IVTDAs fed), dry matter (IVTDDM), organic matter (IVTDOM), neutral detergent fiber (IVTDNDF) and count of protozoans. The digestibility experiments were performed with a DAISY incubator system. Organic acids were not added in the control group and 0.1%, 0.2% or 0.3% FA or MA were added to the experimental groups. Each treatment was replicated 6 times. Samples were incubated for 48 hours. Fumaric acid significantly reduced (P<0.01) all digestibility values of R. pseudoacacialeaves. When FA was applied at 0.l1% to C. sinensis factory wastes, the IVTDOM increased significantly (P <0.05), with the same effect observed for Q. cerris (P<0.01). However, for the addition of 0.1% FA, IVTDAs Fed, IVTDDM and IVTDNDF values decreased significantly (P<0.01). Separately, malic acid did not have a significant effect on the in vitro true digestibility values determined in this study (P>0.05). Rumen protozoan counts decreased for both organic acid applications compared to counts in the fresh rumen contents. Because 0.10% fumaric acid increased the IVTDOM values of both C. sinensis factory wastes and Q. cerrisleaves they can be considered potential alternative feed sources for ruminants.