Le Chatelier's principle has been the subject of much discussion and controversy because it sometimes seems to give the wrong predictions. A classic example is the ammonia gas reaction. The principle is now re-examined, starting from a strict and general derivation given recently. A chemical reaction in a gas mixture is then considered and the conditions are derived under which a chemical reaction will increase the amount of a type of molecule but decrease its chemical potential, a case which has caused confusion. It is demonstrated that there are no exceptions to Le Chatelier's principle if applied to infinitesimal changes of an equilibrium system. However, finite changes may violate the principle. The conditions are given under which this may occur in a gas mixture. The mathematical formulation of the principle is transformed to a form lending itself to quantitative calculations.