All undeveloped films change over time, losing contrast and color balance. This is a gradual chemical reaction, the rate of which is a function primarily of storage conditions. 

• High storage temperatures accelerate change; 

• low temperatures slow it; 

• freezing stops it. 

• Humidity accelerates it; 

• dry storage helps minimize it.

Most film is sold in retail stores, which do not use refrigeration. For that reason, film factories print “process before” dates on their packages. The practice dates from the end of the 19th century, when air conditioning was unknown and refrigeration was uncommon.

The factory “process before” date for film is, typically, two years from the month in which the film was packaged. This is always conservative, because the factory must assume that film on retail shelves is stored relatively warm. The “process before” date of film is actually many months before any deterioration is likely to be visible.

The fact is, films stored at normal room temperature give excellent images long after they have “expired.” The only way a film up to two years past its date will give deteriorated images is if it has been stored warmer than room temperature. 

Film purchased from a specialized photo supplier, who stores film in a refrigerator or freezer, will be of very high quality long past its due date. 

Expired films can continue to give excellent images for many years, if sealed in moisture-proof containers (for example, Tupperware, or zip-lock type bags) and stored in your refrigerator. 

Freezing is even better, extending the life of your films for decades. 

Films can be frozen, thawed, and re-frozen repeatedly with no ill effects.

If you thaw a frozen film, you can probably keep it at room temperature for at least a year, and often longer. It is safest to assume a film should be used within one year of being removed from a freezer.

One caveat: Even frozen films do not last forever. Background radiation does eventually fog films, and freezing does not slow or prevent this. Deterioration will sometimes be noticeable in as few as ten years after the expiry date. Films outdated for fifteen years, stored frozen, may still give reasonably fresh images; by twenty years, noticeable fog is almost guaranteed.

Something else to remember: films change much more rapidly after exposure than before. Exposure causes a physical change to the crystals of silver halide in the light-sensitive layer, which then slowly spreads to nearby crystals in a kind of chain reaction. Exposed film can begin to show loss of contrast and color quality in as few as six to ten weeks. So if you cannot get your exposed film processed promptly, you definitely should refrigerate or freeze it.

Bottom line: have no fear. Buy film that has been properly stored. Store it cool or frozen. Get it developed promptly after exposure. Enjoy it for years.

from http://www.frugalphotographer.com/info-using_expired_film.htm