The daily conditions of prison inmates, i.e. overcrowding and social isolation, have various negative consequences. As one way to palliate the effects of these conditions, animal assisted interventions (AAI) are becoming ever more popular. However, nothing is known either about the potential processes involved or about the potential impact these interventions have on the development of interactions between inmates. Only direct observations of inmates' behaviour during sessions can help understand the potential role of AAI (this triadic situation). We analysed AAI sessions involving 10 male inmates (mean age: 38±5.0 years old), 7 service dogs (4♀3♂, mean age of 18.9±2.9 months old) and one dog handler (female, 40 years old). An observer recorded, using ethological methods, spatial distances between dogs and inmates and between humans, direction of inmates' gazes and their vocal behaviour during each inmate's first one-hour AAI session. Our hypothesis that dogs could be a social catalyst, i.e. facilitator of social interactions between humans, was not supported as each inmate interacted mainly with his own dog. Inmates focused visually on their dog for approximately half the duration of the observation (45.84±5.38%, p< 0.001); it was the main target of their vocal production and it was almost exclusively the only partner with whom they had physical contact (respectively 35.44±8.94% with own dog, 0.76±0.64% with another dog and none with other inmates and the dog handler, p< 0.001). Based on the AAI literature for other human populations, we suggest that the animal/human ratio could be a crucial factor influencing the quality and quantity of AAI interactions.