Theobromine is a water-insoluble, crystalline, bitter powder. Its colour is listed as either white or colourless. It has a similar, but lesser, effect to caffeine, making it a lesser homologue. Theobromine is an isomer of theophylline as well as paraxanthine. Theobromine is categorized as a dimethyl xanthine, which means it is a xanthine with two methyl groups.