It has been proposed that the initial regime of pulp ozonation is a diffusion controlled process, due to the low solubility of ozone and its very fast reaction with lignin. However, whether the process is governed by diffusion on a fibre wall scale (macro diffusion control), on a (micro)fibril scale (micro diffusion control), or by reaction with lignin located on the fibre surface followed by diffusion control is an open question. In order to address this, a 31 kappa Hemlock kraft pulp impregnated with acidified (pH = 2.0) water or acidified (pH = 2.0) ethanol-water (70% w/w) was ozonated at high consistency in a differentially operated packed bed reactor. The cellulose degradation and pulp delignification rates were determined from the change in pulp viscosity and kappa number with ozonation time.