Recently we have reported that polyisoprene/high structured carbon black composites demonstrated excellent chemoresistivity effect [1]. In this study polyisoprene/multiwall carbon nanotube composites (PCNTC) have been elaborated and their electric resistance response to vapour of volatile organic compounds (VOC) was tested. PCNTC has been produced using 2 types of multiwall carbon nanotubes (MWCNT) with clearly different aspect ratio (η) - long MWCNT with η = 12500 and short MWCNT with η = 40 (purchased from « Cheap Tubes Inc »). Additionally to that measurements of electric resistivity temperature dependence showed semiconductor like properties for long MWCNT and metal like properties for short MWCNT. Producing PCNTC with gradually increasing MWCNT concentration, percolation curves of electric conductivity have been obtained and percolation thresholds determined (pc). Percolation threshold is pc = 3.15 and 5.8 for PCNTC with long and short MWCNT respectively. PCNTC response to chemicals stimuli was analyzed by complex measuring technique, which enables simultaneous measurements of electrical resistance, mass and dimensions change of the sample exposed to certain VOC vapour. From obtained results an increase mechanism of composite electrical resistance as well as VOC vapour diffusion behaviour and parameters were evaluated. The MWCNT aspect ratio contribution to the composite VOC vapour sensing was evaluated as well.