Abstract: As both hemispheres are lateralized for speech and language, the objective of this study is to extend the investigation of how the preattentive processing of contour tone changes in Thai monosyllabic words. Twenty two healthy right handed adults participated in this study. This study found that both rising to falling and falling to rising tone changes perception elicited MMN between 212-244 msec with reference to the standard-stimulus ERPs. The rising to falling and falling to rising tone changes elicited a strong MMN bilaterally for native and nonnative speakers of Thai. Source localization was obtained in the Middle Temporal Gyrus (MTG) of the left hemisphere and in the Superior Temporal Gyrus (STG) of the right hemisphere for both groups. Automatic detection of changes in contour tones is a useful index of language universal auditory memory traces.