Is this definitely happening in Lightroom, or are you typing the captions up in Word first?
alexw9 said:In Lightroom, although it could be a setting on my mac that's to blame.
Update:
Try this
Open System Preferences.
Select Personal, then Language and Text.
Choose the Text tab along the upper area of the window.
Uncheck the tick box next to (R)
OR
Uncheck the box next to Correct spelling automatically.
Close the window.