More than half the smartphone apps tested by The Wall Street Journal sent a serial-number-like identifier for the phone to tracking companies.

Some tracking companies use these IDs to create profiles of cellphone users for marketing purposes. The use of these identifiers poses a greater risk than tracking technologies typically used on PC Web browsers, said Heng Xu, an assistant professor of information sciences and technology at Pennsylvania State University. This is because the numbers are difficult or impossible to delete and can be tied to other data, like a person's location at a given moment, she said.

Matching the phone identifier to these other types of information "poses a serious threat to consumer privacy," Ms. Xu said.

There were several types of IDs seen in the Journal's study.

* UDID: The most common identifier the Journal found in its survey was Apple Inc.'s iPhone unique device identifier, or UDID. This combination of 40 numbers and letters is set by Apple and stays with the device forever.