In order to fulfil the targets set by the Kyoto protocol, Belgium established a series of regulations for renewable electricity and put in place a virtual market of green certificates. Their attribution is correlated to the reduction of fossil CO2 emissions. For biomass, emissions occurring during harvest have a significant impact. This paper proposes a model to estimate CO2 emissions during this step and applies it to the chipping of poplar forest residues in Southern Belgium. The factors entering into the CO2 ratio per MWh of biofuel are modelled according to the most influential characteristic of forest residues, i.e. mean initial diameter. The results show that if the diameter of the chipped material increased from 4 to 16cm (factor 4), the CO2 emissions per MWh decreased by a factor 7. This stresses the value of modelling the emissions in order to identify the most critical supply routes for attribution and valuation of the green certificates. (c) 2006 Elsevier Ltd. All rights reserved.