We have developed a method for the synthesis of [2-11C]5,5-dimethyl-2,4-oxazolidinedione ([2-11C]DMO) for use with positron emission tomography to measure regional tissue pH in vivo in man. [2-11C]Dimethyl carbonate (DMC) was prepared from [C-11]phosgene and excess of sodium methoxide in methanol containing dimethyl carbonate as added carrier. The [2-11C]DMC solution was then reacted with 2-hydroxyisobutyramide at 150 degrees C for 10 min to yield, after HPLC separation, [2-11C]DMO with a radiochemical yield of 20-56%. Chemical yields were 78-92%, and specific activity ranged as high as 830 mCi/millimol.