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We theoretically demonstrate a scheme for efficiently generating near-circularly polarized isolated attosecond pulses from mixed gases driven by non-integer frequency ratio two-color multicycle laser fields. It is shown that isolated attosecond pulses with a duration of 310 as can be generated, exhibiting non-zero ellipticity with values reaching as high as 0.9. Further analysis reveals that the ellipticities of these isolated attosecond pulses are attributed to the interference of high-order harmonics generated from different components of the mixture. When the components in the mixture have opposite orbital parity, constructive and destructive interference will occur for left- and right-circularly polarized harmonic components, or vice versa. Additionally, the temporal profile of the isolated attosecond pulse depends solely on the driving fields, while the polarization is determined by the gas mixture. The obtained elliptically polarized isolated attosecond pulses may serve as a promising tool for manipulating ultrafast dynamics in magnetic materials and chiral media. • Highly elliptically polarized attosecond pulses are generated from mixed gases. • We apply the non-integer frequency ratio two-color fields for generating the IAPs. • An IAP with ellipticity up to 0.9 is produced, and its duration is 310 as. [ABSTRACT FROM AUTHOR]