Double differencing of the carrier phases is a popular technique in global positioning system (GPS) surveying for eliminating receiver and satellite clock errors. However, because each global navigation satellite system (GLONASS) satellite transmits a different carrier frequency, it is not straightforward to constrain double difference integer ambiguities from GLONASS carrier phases. Various solutions are reviewed, following the formalism traditionally applied to GPS observations and focusing on the receiver clock errors. Mixed GPS/GLONASS data are used to demonstrate the impact of receiver clock errors on GLONASS double differences carrier phases.