When four carbohydrates were tested against six commonly cited inorganic basal media, post-autoclave pH was highest for carbohydrate-free and sucrose- containing media, and progressively lower for maltose-, P lucose-, and fructose-containing media, respectively, post-autoclave pH for these media without carbohydrates was related to medium buffering capacity. Addition of gelling agents (10 of 11 tested) increased the post¬autoclave pH of MS medium containing sucrose. Neutralized and acid-washed activated charcoal also increased the post-autoclave pH of liquid and agar- solidified MS medium, and the pH changed further during 8 weeks of storage. Changes in medium pH caused by gelling agents, but not charcoal, could be alleviated by adjusting the pH after their addition but prior to autoclaving.