Abstract: Nine artificial diets were prepared by buffer solutions differing in their pH and molarities. They were fed by the fourth larval instar of Spodoptera littoralis for 4 days for testing the ability of alternation in physicochemical conditions of the diets to induce significant changes in nutritional indices, total carbohydrates and fitness of the larvae. pH had a significant reduction on weight gain, RCR (relative consumption rate) and total carbohydrates. On the other hand, the effect of different buffer molarities was significant on weight gain, RGR (Relative Growth Rate), RCR, ECI (efficiency of conversion of ingested food), ECD (efficiency of conversion of digested food) and total carbohydrates. The interaction between the both factors was high as observed in most parameters. AD (Approximate Digestibility) demonstrated that digestion was not adversely affected, which might indicates that midgut has a good buffering system. It was concluded that larvae probably to over come unsuitable physicochemical conditions of the introduced diets, increased their demand on energy through consuming digested or assimilated food like carbohydrates instead of building new tissues, which finally influencing larval performance parameters.