Two vanadium phosphorus oxide precursors doped by Co and Fe have been prepared by a new route consisting of a reduction of VOPO4, 2H(2)O by isobutanol which is known to give a particular morphology of VOHPO4, 0.5H(2)O, developing crystallites in the [110] direction. Their activation under n-butane/air catalytic atmosphere (1.5%) has been compared to the corresponding undoped precursor prepared under the same conditions and used as a reference. The modification of the structure of the VPO materials has been followed on-line by in situ laser Raman spectroscopy. In a first period of the activation, the nucleation of alpha(1)-VOPO4 is favoured for the undoped VPO precursor, while (VO)(2)P2O7 appears in a second period, For the Co-doped VPO precursor, alpha(1)- and alpha(II)-VOPO4 first appear, and then (VO)(2)P2O7 appears. At variance, the Fe-doped VPO precursor promotes the only nucleation of alpha(1)-VOPO4. When comparing with the classical organic route, catalytic performances are markedly improved when the VOHPO4, 0.5H(2)O precursor is prepared via this new route both for the undoped and the Co-doped VPO catalysts, At variance, the Fe-doped catalyst gives poorer performances which have been explained by a high oxidation state for this catalyst (almost (V5+) as alpha(1)-VOPO4). This is confirmed by an analysis of the bulk and surface composition of the final catalysts by XRD and XPS spectroscopy. The role of the two dopants on this new morphology of VOHPO4, 0.5H(2)O is then quite different at variance with what was observed on the same precursor prepared by the classical route of reduction of V2O5 by isobutanol (see B. Abdelouahab et al., 1995, J. Catal., 157, 687, The present study demonstrates that the preparative route for the formation of doped VPO precursors is most important in controlling the VOPO4/(VO)(2)P2O7 dispersion and catalytic performance of the final catalyst.