Immutable codes have the property that information recorded with them on write-once memories such as digital optical discs cannot be changed. Write-once memory permits changing a0into a1, but once a1is written it cannot be changed back into a0. Most commonly used codes do not have the property of immutability. After a very general definition of immutability is given, algorithms which test a given code for immutability are developed. Algorithms are presented for fixed-length codes as well as for variable-length codes.