The influence of the cigarette paper on carbon monoxide deliveries is well known. The two major mechanisms are air dilution and CO diffusion through the cigarette paper. If dilution is directly related to the air permeability of the paper, the CO diffusion is on the other hand strongly related to the porous structure of the paper. We demonstrated last year that the measurement of Diffusional Conductance Index (DCI) and measurement of gas diffusivity through the cigarette paper are efficient tools to evaluate the capacity of papers to diffuse CO. The objective of this presentation is to show the benefit of the use of these tools: We will first explain how the measurement of diffusivity can help to discriminate papers having identical characteristics, including especially the air permeability, but leading to different CO deliveries. Examples will be given. Then, we will focus on the influence of paper weight (g/m²), filler content (%), filler weight (g/m²) and fibre weight (g/m²) on the papers capacity to diffuse CO. Impact on CO deliveries and CO/tar ratio will be discussed. We will finally demonstrate using examples that both the measurement of Diffusional Conductance Index of the papers or the measurement of gas diffusivity through the cigarette paper is a key approach to develop cigarette papers appropriate to low CO delivery cigarettes.