People with autism spectrum disorder (ASD), can have difficulties in emotion processing, including recognising their own and others' emotions, leading to problems in emotion regulation and interpersonal relating. This study reports the development and piloting of the Client Emotional Processing Scale-Autism Spectrum (CEPS-AS), a new observer measure of four interrelated aspects of emotional processing: emotion recognition, self-reflection, cognitive empathy, and affective empathy. Results showed good interrater reliability (alpha: 0.69-0.91), while inter-dimension associations were high (r = 0.66-0.82). The measure was able to detect significant differences on the four dimensions across a short-term humanistic-experiential group therapy. The CEPS-AS shows promise as a potential addition to current self-report instruments measuring empathy or emotion processes in individuals with ASD.