We report simultaneous measurements of fuel consumption, driving patterns, and CO2, CO, and NOx emission factors for diesel passenger buses under real operating conditions in high altitude cities (>2000 masl) and in mountainous regions with an average road grade of 4%. These measurements were obtained using a Portable Emission Measurement System (PEMS) monitoring a sample of 15 buses during eight months of daily operation. For a city with a high level of vehicular traffic (Mexico City), we obtained an average fuel consumption of 0.41 L/km and emission factors of 965.8, 41.4, and 5.3 g/km for CO2, CO, and NOx, respectively. Fuel consumption and CO2 emissions were within expected values. However, CO emissions were approximately three times higher while the NOx emission were half of the reported values in the literature for buses of similar technology working at low altitude.