Doubly oriented iodine-doped nylon 6 (iodine-nylon 6 complex) was prepared by immersing nylon 6 in an I2-KI aqueous solution and was studied using X-ray diffraction. The intensities in the X-ray diffraction pattern from the iodine-doped nylon 6 vary with ageing time after doping. The variation is attributed to the rearrangements of polyiodides in the complex. In the complex two kinds of ’one-dimensional’ ordered structures give streaks on the layer lines, and a three-dimensional ordered structure (complex crystal) gives Bragg reflections. Just after doping an unstable ’one-dimensional’ structure emerges. This subsequently transforms into a stable ’one-dimensional’ structure and/or the three-dimensional complex crystal with ageing. The arrangement of polyiodides in the complex has been determined. The unstable ’one-dimensional’ structure is incommensurate with the complex crystal; a structural model based on co-ordination of I2 with I3- is proposed on the basis of the intensities of the unstable streaks vanishing with ageing. The complex crystal is concluded to include I3- ions inclining at an angle to the chain axis of nylon 6. The stable ’one-dimensional’ structure contains I5- ions which are oriented parallel to the chain axis.