Abstract Aviation English, one variety of English for special purposes (ESP), is used in aviation communication worldwide. However, since being adopted as a common medium for radio communication between pilots and controllers in the 1950's, language problem shave been reported involving native and non-native English speaking pilots and controllers. The types of language problems identified in earlier studies include: a) the accent of pilots and controllers, b) the use of ambiguous words or phrases, and c) the use of non-standard ICAO phrases. In this study, the researcher distributed questionnaires to and interviewed pilots and air traffic controllers at the Macau International Airport to determine the types of problems pilots and ATCs had using English, their perception of their English ability, their use of L1 in radiocommunications, and their adherence to ICAO standard phraseology. Real-time communications between ATCs and pilots were analyzed to determine the effect of message length on message comprehension. The findings of the study were that pilots and ATCs found accent to be an important problem. Language proficiency influenced pilot and ATC perceptions of their communication abilities; under certain conditions, pilots and ATCs use L1; pilots and ATCs generally adhere to ICAO standard phraseology, and controller initiated messages of more than three aviation topics affected pilot comprehension of the messages