The effect of co-cracking of high-density polyethylene (HDPE) or its pyrolysis wax together with vacuum gasoil (VGO) has been studied. The aim is to determine the content, nature, and location of coke while analyzing the impact on product distribution in the process. Four different feeds were used (VGO, VGO + 5 wt% HDPE, VGO + 20 wt% of wax, and 100 wt% wax) in experiments performed in a laboratory-scaled riser simulator, with similar conditions to those of the fluid catalytic cracking (FCC) unit: equilibrium catalyst; 530 degrees C; catalyst/feed mass ratio, 5; contact time, 6 s. The results of the different characterization techniques of the deactivated catalyst and the coke indicate that the catalyst fouling decreases notably by incorporating these waste streams, changing its nature to a more aliphatic (olefinic) one and its location towards the zeolite micropores. On the contrary, the coke formed from the VGO is more evolved and mainly constituted by polyaromatic components deposited on the mesopores of the catalyst matrix. Product distribution is also affected, increasing the yield of light cycle oil and keeping a relatively similar gasoline yield, with greater olefinity and lower aromaticity. Thus, the FCC shows great perspectives to valorize polyolefins, present in waste plastics, at great scale.