Observed rotational-vibrational transitions of HI dimer in the geared bending mode, centered at 511.9 GHz, are reported. This similar to 50 kHz spectrum was recorded using a co-axially configured pulsed jet submillimeter spectrometer and hyperfine structure of R(J) and P(J) transitions from the quadrupole moments of iodine nuclei are completely resolved for low-J transitions. Analysis of hyperfine patterns was carried out using a theoretical approach accounting for the large amplitude motion effects and hyperfine matrix elements within and between vibrational states. The submillimeter analysis is consistent with a vibrationally averaged ground state R-cm = 4.56372(1) angstrom and average bending angle theta = 46.405(1)degrees. (C) 2009 Elsevier B. V. All rights reserved.