The Modified-Classroom Observation Schedule to Measure Intentional Communication (M-COSMIC) was developed as an ecologically valid measure of social-communication behaviour, delineating forms, functions, and intended partners of children's spontaneous communication acts. Forty-one children with autism spectrum disorder (ASD) aged 48-73 months were filmed within small-group settings at school. Communication behaviours during a 5-min teacher-led activity and a 10-min free-play session were coded from videotape. Inter-rater reliability was high. Many M-COSMIC codes were significantly associated as predicted with Social and Communication domain scores on the Autism Diagnostic Observation Schedule (ADOS) and with scores on standardised language assessments. Agreement was more variable, however, at the level of individual M-COSMIC codes and ADOS items. Higher rates of responding, compliance behaviours and following pointing gestures and gaze occurred during the more structured teacher-led activity, compared to the free-play. Results demonstrate preliminary construct validity of the M-COSMIC, showing its potential to describe and evaluate spontaneous social-communication skills in young children with ASD for research and applied purposes. (Contains 5 tables and 4 figures.)