The invention concerns a data compaction method and system for an intermediate program. The method consists in searching the program (1000) for identical sequences (Si) and counting Ni number of occurrences of each sequence (Si), a comparison test (1001) to find the superiority of a function f(Ni) to a reference value enables to generate (1003) a specific instruction of a specific code (Ci) with which the sequence (Si) is associated, replacing (1004) each occurrence in the sequence (Si) by the specific code (Ci) in the intermediate program to create a compacted intermediate program (FCC) with which an executing file (FEX) is associated. The invention is applicable to multiple application portable objects such as microprocessor cards, onboard systems of the like.