Probably the best and simplest page I've seen on maturation:
http://www.20pacesdistillery.com/maturation.html" onclick="window.open(this.href);return false;" rel="nofollow
EXTRACTION
Carefully selected seasoned, toasted and charred oak imparts a wide a range of flavors to the maturing spirit, including vanillins, lactones, spice characters and tannins. The larger the surface area of the oak to the volume of the spirit, the more of these flavors are released. This process is also temperature dependent.
OXIDATION
Barrels are semi-permeable containers allowing volatile components to escape and air to enter. Oxygen from the head space dissolves in the distillate and reacts with compounds extracted from the oak or originating from the distilled spirit, creating novel flavors.
EVAPORATION
Because barrels are semi-permeable, about 8% of its content is lost during the first year and about 3% in the following years. Besides allowing more air to enter the barrel, components with lower boiling points and higher vapor pressure leave the barrel which further improves the final product.
REACTION
Reaction rates are temperature dependent. An increase in temperature will increase reaction rates and vice versa. Empirically, a temperature increase of 10 deg Celsius will double the reaction rate. Reaction rates also depend on the type of reaction and the amount of the respective reagents.
