Caffeine ingestion and fluid balance: a review
The results of this study are a little surprising for me. Basically, although caffeine is known to have a diuretic effect, this review suggests that this effect is only temporary and only seen if the person is an occasional caffeine ingester.
Caffeine ingestion and fluid balance: a review – J Hum Nutr Diet, Vol 16, Issue 6, pp. 411-420 (Abstract).