An mp3-standard bitstream is formatted into a sequence of fixed-length data frames. These include headers, side information, main information and a remaining data field without generally defined information denoted as `ancillary data`. The mp3PRO format is an extension of the mp3 format, wherein the additional mp3PRO data are transferred in the ancillary data fields. In various applications, e.g. Internet music search machines, a necessity arises for a fast determination of the bitstream types. Such determination is normally executed using an mp3PRO decoder. However, because the frame header does not contain a corresponding pointer to the start address of the ancillary data field, an mp3PRO decoder must first completely decode at least one data frame according to the mp3 standard in order to find the end address of the mp3 data and thereby the following start address of the mp3PRO data in that data frame. Thereafter the mp3PRO decoder must examine the data following in the data stream for characteristics that are typical for mp3PRO additional information. The invention discloses how the bitstream type can be determined without using mp3 decoding and without using an mp3PRO decoder.