Electrical conductivity response of polyaniline/zeolite composites towards CO is investigated in terms of dopant type, dopant concentration, zeolite LTA content and zeolite pore size. Both MA and HCl doped polyanilines respond with comparable magnitudes towards CO; the latter responses are slightly smaller for the same doping level. Addition of zeolite 4A reduces the electrical conductivity response but improves the sensitivity towards CO with increasing zeolite concentration up to 40% w/w. This concentration is evidently below the percolation threshold value, which is estimated to be above 50% w/w. Composite with zeolite 3A has a comparable sensitivity value relative to that of pure polyaniline. Composites of 4A and 5A have greater sensitivity values over that of the pure polyaniline at the CO concentration range between 16 and 1000 ppm. Zeolite 5A is the most effective mesoporous material in promoting interaction between CO and polyaniline because of its largest pore size of 5 A, relative to the zeolite 3A and 4A which have the pore sizes of 4 and 3 A, respectively. (C) 2004 Published by Elsevier Ltd.