Display omitted •Mechanochemically accelerated vapour–assisted aging in the synthesis of (poly)oxalatomolybdates.•Transformation kinetics of the amorphous intermediates into the crystalline products.•In situ reaction monitoring by PXRD. Mechanochemically accelerated vapour–assisted aging and transformation from the amorphous precipitate by aging to the crystalline products were demonstrated to be the efficient methods to synthesize crystalline hydrate and non-hydrate (poly)oxalatomolybdates, Co(NO2)(NH3)5MoO3(ox)H2O·H2O, Co(NH3)62Mo4O11(ox)4(H2O)·6H2O, Co(NH3)64Mo4O8(μ-O)4(ox)4MoO3(ox)(H2O)2·10H2O, Co(en)34MoO2(μ-O)(ox)2MoO3(ox)(H2O)2·7H2O and Co(ox)(en)22Mo2O5(ox)2(H2O)2·4H2O, respectively. We have also studied the transformation kinetics of the amorphous intermediates into the final crystalline oxalatomolybdates in an aqueous slurry at room temperature by in situ powder X-ray diffraction (PXRD) method coupled with numerical methods. Using the principal component analysis, PXRD data tensors were reduced to only one principal component. Reaction profiles were built and total times for completion were determined.