Microstructure profiles of temperature, conductivity, and velocity shear during the Arctic Internal Wave Experiment (AIWEX) in March--April 1985 in the Beaufort Sea are used to investigate the thermodynamic processes in a diffusive thermohaline staircase. The staircase occurs between depths of about 320 and 430 m, above the core of the relatively warm, salty Atlantic water, where the mean temperature and salinity are increasing with depth. Individual isothermal layers can be tracked for at least several hours, suggesting a horizontal length scale of several hundred meters or more, assuming a typical relative velocity of 0.01 m s-1 at this time. Over the depth range 320--430 m the mean (average over several steps) density ratio ⟨R&rgr; ⟩=β⟨Sz >/α⟨Tz ⟩ varies between 4 and 6, while the typical temperature difference between layers decreases from 0.012¿ to 0.004 ¿C. The mean thickness of the layers also varies, from 1 m at 320 m depth to 2 m at 430 m. The relationship proposed by Kelley (1984), relating layer height to ⟨N2 ⟩, ⟨R&rgr; >, and molecular properties of the fluid, overestimates the mean layer thickness by about a factor of 2. The variability of staircase characteristics suggests that oceanic staircases may rarely, if ever, be steady state, but in general be slowly evolving from previous perturbations. Heat fluxes estimated from laboratory-based flux laws, involving R&rgr; and ΔT, are in the range 0.02<FH <0.1 W m-2 , which is in agreement with the molecular heat fluxes through the maximum interfacial temperature gradients. There are no interfaces where the kinetic energy dissipation rate (averaged over 0.5 m) exceeds the lower limit for diapycnal mixing, 24.5&ngr;N2 . ¿American Geophysical Union 1987