Plants are an integral part of nature. Ever since the birth of man the plant kingdom has provided him with food, shelter and drugs. Plants are sophisticated factories where a variety of chemical compounds are manufactured. They are now recognized for the production of a large variety of chemicals. These plant-derived products are used as medicines, pesticides, perfumes, fragrances, etc. The Indian system of medicine, Ayurveda, makes use of plant kingdom very effectively and has stood the test of time. Phytochemical studies have become more encouraging with the advent of modern spectroscopic techniques and instrumentation. There are many plants on earth which lies unexplored in the field of medicine or Science. The work presented in this thesis deals with the chemical investigation of thorns and leaves of Canthium parviflorum and air-dried Biophytum sensitivum, the analyses of essential oils from Zanthoxylum rhetsa seeds, Anisochilus carnosus spikes and roots and air-dried Biophytum sensitivum from South India. The chemical transformation and the antimicrobial activities of Zanthoxylum rhetsa seed essential oil have also been studied.