Abstract The growth of fourth-instar Manduca sexta larvae on nutrient-rich artificial diets is significantly affected by the characteristics of the buffer system present in the diet. An increase in diet buffer concentration or buffering capacity can cause decreases in total larval weight gain, relative growth rate, net growth efficiency and larval lipid content, and increases in the length of the instar, respiration rate, and the amount of assimilated food allocated to energy metabolism. We conclude that there is a significant metabolic cost associated with processing a diet with a high buffer concentration or buffering capacity. Within the pH range examined in this study (4.4–5.5), pH has a less pronounced effect on herbivore growth parameters, and presumably also on fitness, than do buffer concentration and buffering capacity. These results demonstrate that foliar buffer systems are potentially important determinants of the nutritional value of foliage to insect herbivores.