The main goal of this work consists in the research about the nowadays actual capabilities of X-ray powder diffraction to solve complex organic structures. At this respect, we consider here two organic compounds: caffeine and nicotine, whose biological activity led us to include them as stimulant drugs. Since it is very difficult to obtain single crystals of these substances, suitable for the four-circle diffractometer, we have adopted for its structural determination the pathway starting with their respective X-ray powder patterns.