The purpose of this article was to describe a Finnish research project concerning communicative spontaneity in pupils with autism spectrum disorder (ASD). ASD is a behavioural syndrome which is neurobiological in origin and which involves atypical developmental dysfunctions in the brain. The essential features are persistent impairment in reciprocal social communication and social interaction, and restricted, repetitive patterns of behaviour, interest, and activities. However, an important part of flexible interaction is functional communication between partners, and spontaneity is a critical aspect of functional communication. Communicative spontaneity can be assessed by examining the relationship between environmental antecedents or cues (prompts) and communicative acts. In this study spontaneity is viewed along a continuum, it is, all communicative acts are considered to have some degree of spontaneity and all communication is environmentally prompted or cued. In this research a model of supportive prompts for spontaneity was applied in practice. The model in question is Carter and Hotchkis's four-level antecedent model. The study explored the supportive prompts ethnomethodologically. In the study described six video recordings (each about 30?min) from authentic classroom situations were categorised with the four-level antecedent model. These videos are unusual in Finnish context in that they involve only the teacher and her six pupils with ASD; no helpers are present in the classroom. This study indicated that the four-level antecedent model is a powerful means of analysing the dimensions of spontaneous communication, but the sensitivity of the model can be increased. The authors' suggestion is that the modification of the model may be accomplished by adding new categories (such as non-acting or prompt-seeking behaviour) or editing current categories. The modified four-level antecedent model is more applicable to help teachers develop spontaneity in the communication of pupils with ASD.