I use a 1/2" breaker with a 3/8" adapter on it.Thank you all for the responses I have received regarding the serpentine belt replacement. I have been afraid to do this. In addition, do you need a special tool to release the belt tensioner?
A long enough 3/8" breaker bar would do the job fine (mine barely fits).
The Serpentine Belt tool I bought doesn't fit good enough, alas.