The invention relates to any conversions in which data of a certain source format are converted into data of a certain target format. The conversions may also be bi-directional. The invention includes a method for performing data structure conversions, wherein a data structure comprises at least two elements located in a predetermined order in the data structure. The data structure is defined by using a definition language, and it is represented as a source bit string. During the conversion the source bit string is converted into a target bit string. The method for performing the data structure conversion handles the data structure elements in the same order as they are located in the data structure without parsing the source bit string. The avoidance of the parsing makes the method more efficient than the prior art methods. In addition, the method decodes each data structure element and encodes it directly into the target bit string. Also the avoidance of unnecessary copying of data makes the method efficient.