The synthesis of pillar[5]arenes from p -dialkoxybenzene and formaldehyde in the presence of iron(III) chloride and tetramethylammonium chloride under mild reaction conditions was investigated in detail. For the fi rst time, the presence of linear intermediate oligomers and their conversion into pillar[6]arenes was demonstrated. Incorporation experiments were carried out using methoxy- and ethoxy-functionalized pillar[5]arenes that were reacted respectively with 1,4-diethoxy- and 1,4-dimethoxybenzene, observing the formation of the corresponding co-pillar[6]arene derivatives, PMe[5]Et[1] and PEt[5]Me[1]. An experiment in which methoxy- and ethoxy-functionalized pillar[5]arenes were reacted together led to the major formation of scrambled P[5] derivatives characterized by a bell shape distribution of ethoxy and methoxy aromatic units. Co-Pillar[6]arenes P[6] were also detected by ESI-MS in minor amounts with a similar symmetrical distribution of substituents that allowed the proposal of multiple cleavage events occurring on the same macrocyclic ring forming in solution a series of smaller fragments, whose recombination leads to the observed product distribution of P[5] and P[6] derivatives.