In this paper, the penetration of water and black liquor into overthick and pre-treated overthick chips (cracked and re-chipped according to thickness) was studied with a laboratory impregnator to investigate ways to improve penetration. Overthick chips were divided in three categories: knot-containing chips, chips without knots and original overthick chips. The results of the penetration experiments were evaluated in terms of the penetration degree and liquor uptake. The results showed that cracking and re-chipping with a sizer are very effective ways to improve penetration in overthick chips. These pre-treatments gave higher and faster penetration than that achieved with normal pulpwood chips. With knot and original overthick chips, penetration degree values were at a lower level compared to normal pulpwood, but still over 92-97.5% depending on penetration conditions. Liquor uptake and penetrated chemical charge were almost 50% lower with overthick, knot and sizer chips compared to normal, knot free and cracked chips, the main reasons being high basic density and in the case of sizer chips high initial moisture content. Cracking decreased initial chip moisture content and therefore liquor uptake was about 60% higher with these cracked chips than with sizer chips. Although a high penetration degree (> 95%) was achieved quite quickly (within 20 min) with all chip grades, penetrated EA-charge was very low. In normal cooking conditions penetration is much less effective way to transport chemicals into chips than diffusion, but a high penextration degree is a prerequisite for fast diffusion. Therefore chips should be impregnated with conditions preferable to a fast diffusion of cooking chemicals. The penetration method user proved to be suitable only for water penetration studies. If liquors other than water are used then diffusion of chemicals and dissolution of wood material are involved in the mass transfer process at the same time as penextration. This means that some changes to the calculations and additional information about impregnation are needed if penextration of liquors alone is to be studied.