Ink trappings were studied by using polyethylene terephthalate (PET) film with black inks for offset proofing and synthetic paper. By observing printed matter from reverse side through the PET film, we detected many air bubbles in the ink layer and between the ink layer and the PET film. They are classified roughly to two groups, small number of large ones (φ = 2 - 5 μm) and many small ones (φ = 0.5 - 1.0 μm). The former ones were fixed air bubbles during the trapping. The latter ones decreased according to increase the amount of ink trapped (y). Because number of the air bubbles (Nair bubble) increased with increasing the ink distribution time, they seemed to be yielded by suspension of air into the ink layer during ink distribution. By observing printed surface, we also detected many ink peaks (immediately after the trapping) and pinholes (at 24 h). The numbers of the ink peaks and pinholes (Nink peak and Npinhole, respectively) decreased also with increasing the y value and increased with increasing the ink distribution time. We studied effects of nip width on these values (distribution time = 2 min.; nip width = 2, 3 and 4 mm). The Nair bubble value decreased with increasing nip width contrary to increase the Nink peak and Npinhole values. The effects can be represented by differences in the values of 2 and 4 mm nip widths. At y = 2 gm-2, the difference in the Nair bubble value is about one third (synthetic paper ink) or a half (offset proofing ink) of the difference in the Nink peak values.