Atom transfer radical polymerization (ATRP) of acrylates in ionic liquid, 1-butyl-3-methylimidazolium hexaflurophospate, with the CuBr/CuBr2/amine catalytic system was investigated. Sequential polymerization was performed by synthesizing AB block copolymers. Polymerization of butyl acrylate (monomer that is only partly soluble in an ionic liquid forming a two-phase system) proceeded to practically quantitative conversion. If the second monomer (methyl acrylate) is added at this stage, polymerization proceeds, and block copolymer formed is essentially free of homopolymer according to size exclusion chromatographic analysis. The number-average molecular weight of the copolymer is slightly higher than calculated, but the molecular weight distribution is low (M-w/M-n = 1.12). If, however, methyl acrylate (monomer that is soluble in an ionic liquid) is polymerized at the first stage, then butyl acrylate in the second-stage situation is different. Block copolymer free of homopolymer of the first block (with M-w/M-n = 1.13) may be obtained only if the conversion of methyl acrylate at the stage when second monomer is added is not higher than 70%. Matrix-assisted laser desorption/ionization time-of-flight analysis confirmed that irreversible deactivation of growing macromolecules is significant for methyl acrylate polymerization at a monomer conversion above 70%, whereas it is still not significant for butyl acrylate even at practically quantitative conversion. These results show that ATRP of butyl acrylate in ionic liquid followed by addition of a second acrylate monomer allows the clean synthesis of block copolymers by one-pot sequential polymerization even if the first stage is carried out to complete conversion of butyl acrylate.