The abundance of CO 2 in comets has been derived using CO Cameron band (a 3 Π→X 1 Σ + ) emission assuming that photodissociative excitation of CO 2 is the main production process of CO(a 3 Π). On comet 1P/Halley the Cameron (1-0) band has been observed by International Ultraviolet Explorer (IUE) on several days in March 1986. A coupled chemistry–emission model is developed for comet 1P/Halley to assess the importance of various production and loss mechanisms of CO(a 3 Π) and to calculate the intensity of Cameron band emission on different days of IUE observation. Two different solar EUV flux models, EUVAC of Richards et al. (1994) and SOLAR2000 of Tobiska (2004), and different relative abundances of CO and CO 2 , are used to evaluate the role of photon and photoelectron in producing CO molecule in a 3 Π state in the cometary coma. It is found that in comet 1P/Halley 60–70% of the total intensity of the Cameron band emission is contributed by electron impact excitation of CO and CO 2 , while the contribution from photodissociative excitation of CO 2 is small (20–30%). Thus, in the comets where CO and CO 2 relative abundances are comparable, the Cameron band emission is largely governed by electron impact excitation of CO, and not by the photodissociative excitation of CO2 as assumed earlier. Model calculated Cameron band 1-0 emission intensity (40 R) is consistent with the observed IUE slit-averaged brightness (37±6 R) using EUVAC model solar flux on 13 March 1986, and also on other days of observations. Since electron impact excitation is the major production mechanism, the Cameron emission can be used to derive photoelectron density in the inner coma rather than the CO 2 abundance.