Phase I metabolism is the first phase in elimination. Simply stated, it is the oxidation, reduction or hydrolysis of a foreign compound. It generally alters the substrate to be more polar. It does this directly and indirectly with the ultimate goal of facilitating excretion by the kidneys. Some compounds can be eliminated solely by Phase I enzymes, those remaining will have small polar molecules attached to them by the Phase II enzymes. For the Phase II enzymes to perform their task, however, there must be an oxygen attached which is often only present thanks to the Phase I enzyme oxidation.