In the present study we have investigated the ionization threshold behavior of neon cluster ions Nen+ (with n=2-6) formed upon the electron ionization of small neon clusters. The clusters were formed by supersonic expansion of cold neon gas through a pinhole nozzle. The appearance energies for these cluster ions were subsequently determined using a nonlinear least-squares fitting procedure. The obtained thresholds turned out to be only slightly lower than the ionization energy of the single neon atom. The present results are compared with previous electron impact and photoionization results. In addition, we investigated the electron ionization of neon clusters doped with CO2 molecules. The mass spectrum at the electron energy of 70 eV showed predominantly bare cluster ions of CO2. Charged clusters with fragments of CO2 were observed as well, though in weaker abundance than the intact cluster ions. The relative abundance of these fragment ions was different from the ratios previously reported for electron ionization of bare CO2 clusters and indicated increased formation of (CO2)4O2+ due to a neon matrix effect. We further investigated the ionization mechanisms in the cluster by measuring the ion yields of (CO2)4+ as a function of the electron energy close to the threshold. Direct ionization by the incoming electron, Penning ionization, and charge-transfer ionization were identified as possible processes, with varying contributions for different initial mean neon cluster sizes.