As a central component of the smart city, sensor infrastructures locate and measure a wide range of variables in order to characterise the urban environment. Perhaps the most visible expression of the smart city, sensor deployment is a key equity concern. As new sensor technologies and Big Data interact with social processes, they have the potential to reproduce well-documented spatial injustices.Contrary to promises of providing new knowledge and data for cities, they can also create new gaps in understanding about specific urban populations that fall into the interstices of data collectionsensor deserts. Building on emerging data justice debates, specifically considering distributional, recognition, and procedural forms of injustice, we conceptualise and analyse sensor deserts through two case studies, Newcastle's Urban Observatory (UK) and Chicago's Array of Things (USA). Open sensor locations are integrated with small-area, socio-economic data to evidence the demographic configuration and spatialities of sensor deserts across each city. We illustrate how the structural processes via which inequality is reinforced by smart agendas manifest as uneven social and spatial outcomes. In doing so, the paper opens up a new conceptual space in which to consider what it means (not) to count in the smart city, bringing a demographic perspective to critical debates about smart urbanisms.