Carrier transport properties of AlGaN/GaN heterostructures have been analyzed with the quantitative mobility spectrum analysis (QMSA) technique. The nominally-undoped Al0.15Ga0.85N/GaN sample was grown by metal-organic vapor phase epitaxy. Variable-magneticfield Hall measurements were carried out in the temperature range of 4-160 K and magnetic field range of 0-6.6 T. QMSA was applied to the experimental variable-field data to extract the concentrations and mobilities associated with the high-mobility 2DEG and the relatively lowmobility bulk electrons for the temperature range investigated. For temperatures below 100 K the calculated mobility and carrier density values were close to the experimental results. No bulk conduction was observed in this temperature range. At 160 K, QMSA results show that parallel conduction in 3 mm thick GaN layer started to affect the average electron mobility.