Abstract 3YSZ green layers approximately 10 μm thick were screen-printed onto 3YSZ substrates and their constrained sintering kinetics were measured at 1100–1350 °C using an optical dilatometer. The densification rates of the same powder in the form of pellets and free-standing films were also measured. The constrained densification rate was greatly retarded compared with the free densification rate at a given temperature and density. The retardation increased with increasing density and temperature and could not be properly accounted for by existing theories of constrained sintering. As a result the apparent activation energy is much lower for constrained sintering (135 ± 20 kJ mol −1) than for free sintering (660 ± 30 kJ mol −1). It is proposed that this is because the constrained microstructure exhibits larger and more widely separated pores at the higher temperatures.