This paper presents a novel microstructure for an ethene/1-olefin copolymer. Ethene/4-methyl-1-pentene copolymers were found to be essentially made of sequences of ethene alternating with sequences of 4-methyl-1-pentene, with a minor amount of isolated ethene units distributed in the 1-olefin sequence. This microstructure is produced by highly isospecific catalytic sites and can be explained on the basis of a penultimate unit effect in copolymerization. Two isospecific organometallic complexes were used to prepare said copolymer: a sterically hindered, highly regio- and stereospecific metallocene, rac-[methylenebis(3-tert-butyl-1-indenyl)]zirconium dichloride [rac-H2C-(3-'BuInd)(2)ZrCl2] (TBI), and the prototypical moderately isospecific hydrogenated metallocene without any substituent on the indenyl ligand, rac-ethylenebis(tetrahydroindenyl)zirconium dichloride [rac-(EBTHI)ZrCl2] (EBTHI). As a comparison, copolymers were also prepared with the so-called "constrained geometry" half-sandwich complex, {eta(1):eta(5)-[(tert-butyl-amido)dimethylsilyl)(2,3,4,5-tetramethyl-1-c yclopentadienyl)}-titanium dichloride [Me2Si(Me4Cp)(N-'Bu)TiCl2] (CG). Comonomer sequences at the triad level were determined through C-13 NMR analysis, and reactivity ratios of copolymerizations were elaborated through a statistical method, investigating the effect of ultimate and penultimate inserted comonomer units. Values of r(1)r(2), products larger than I were calculated for copolymerizations from both the isospecific metallocenes, with relatively high values of both r(1) and r(2). The investigation of the penultimate-unit effect brought to calculate high values of both r(11) and r(22) and unprecedented contemporary low values of r(12) and, surprisingly, r(21) to indicate the occurring of comonomer sequences and the unusual absence of alternate comonomers. In particular, the low r(21) value indicates that the 4-methyl-1-pentene unit in the penultimate position causes a remarkable decrease of the ethene reactivity, with the insertion of a minor amount of isolated units in 4-methyl-1-pentene sequences. A product of reactivity ratios r(1)r(2) close to 1 was obtained with CG as the catalyst precursor, not only confirming the tendency of this organometallic complex to promote almost random ethene/1-olefin copolymerizations, already observed with propene, as the 1-olefin, but also suggesting that the isospecificity of the catalytic site plays a key role in affording the novel copolymer microstructure. With 4-methyl-1-pentene as the comonomer, for the first time long 1-olefin sequences are observed in an ethene copolymer obtained with a catalytic system poorly isospecific in propene homopolymerization, such as EBTHI This seems to suggest that the branched comonomer 4-methyl-1-pentene contributes to the formation of a highly isospecific site, whose stereospecificity is once more confirmed to arise from the cooperation of the catalyst and of the growing chain. These findings seem also to indicate that when a catalytic site becomes highly isospecific, it is likely to form sequences of both comonomers in an ethene/1-olefin copolymer.