Defibration and fibre development patterns were investigated for the novel ATMP refining process, based on pilot scale trials with Norway spruce chips. ATMP refining with different chemical agents was compared to RTS refining with and without Impressa-finer (RT) and Fiberizer (F) pre-treatment. RT-F pre-treatment significantly improved both initial defibration, axial fibre splitting and fibre flexibility, compared to RTS primary stage refining without pre-treatment. Both types of investigated ATMP process chemistry - hydrogen peroxide combined with magnesium hydroxide under alkaline conditions (P) or acid sodium bisulphite (S) added to the primary refiner dilution water - further improved the fibre separation of RT-F pre-treated wood during primary stage RTS refining. This is largely attributed to enhanced fibre swelling. S-treatment facilitated frequent fibre separation within or close to the S2 wall layer, yielding extremely low shive levels and well-fibrillated, thin-walled fibres early in the process. S-treatment also rendered stiffer fibres, which made them susceptible to breakage, axial splitting and internal delamination. P-application is proposed to affect primarily the interior layers of the fibre walls, facilitating rapid fibre wall swelling towards the lumen, fibre softening and flexibilization.