The supply of high-concentrate diets results in the reduction in ruminal pH and the development of acidosis. Because the reduction of pH occurs at the same time as the diet is changed, the effects are confounded. For example, high-concentrate diets ferment towards propionate, and low rumen pH also results in greater propionate production. Which factor is responsible for the increase in rumen propionate? The objective of this study was to determine the effects of rumen pH and the type of diet (D) on rumen microbial fermentation, with the aim of developing simple mathematical model to describe these effects.