It has recently become possible to characterize the atmospheres of some extrasolar planets via transit spectroscopy, which is a crucial stepping stone towards future studies of Earth-like planets. For hot Jupiters in particular, potassium has been predicted to be one of the strongest sources of opacity at optical wavelengths. Indeed, it has been previously detected in the atmospheres of one of my planets of interest (XO-2b). I presented narrow-band spectrophotometric observations of XO-2b as well as TrES-2b made with the 10.4-meter Gran Telescopio Canarias to confirm/search for absorption due to potassium in their atmospheres. I discussed the results from my search for potassium absorption in these planetary atmospheres as well as recent progress made in characterizing the atmospheres of hot Jupiters with other large ground-based telescopes.