Measurements of the far-infrared reflectance of URu2Si2 between 10 and 720 cm -1 have been made at temperatures from 2 to 90 K. Above the coherence temperature the optical conductivity increases monotonically with increasing frequency and at lower temperatures the development of the narrow mode responsible for the high dc conductivity is clearly observed. In the antiferromagnetic state an energy gap with a size between 46 and 65 cm" 1 is observed whose shape is reminiscent of the energy gap observed in the spin-density-wave state of Cr. PACS numbers: 78.30.Er, 72.15.Qm, 75.30.Mb URu2Si2 has attracted interest as the first heavyfermion metal shown to exhibit both superconductivity and magnetic ordering. Specific-heat measurements indicate that the effective mass of the electrons in this material is enhanced at low temperature by a renormalization parameter, A, , of the order of 25. l The transport properties of URu2Si2 are also similar to other heavyfermion metals. Above 70 K the dc resistivity decreases with increasing temperature 2 in a manner similar to that observed in metals containing isolated magnetic impurities. This similarity to the Kondo problem suggests that the high-temperature behavior of URu2Si2 might be related to the scattering of conduction electrons by isolated magnetic impurities. 3 Below 70 K lies the coherent regime which is the temperature range in which the resistivity drops rapidly with decreasing temperature. 3 Reflectance measurements are presented here both above and below the coherent temperature, r co , the 70-K boundary between the single-impurity regime and the coherent regime. At 17.5 K there is a resistivity anomaly similar to the one at the spin-density-wave transition (SDW) in Cr (Ref. 2) and neutron scattering has confirmed that this is an antiferromagnetic transition. 4 Far-infrared measurements probe the energy gap associated with this transition. A coexisting superconducting transition near 1 K is too low in temperature to observe in the far infrared. 2 The single crystal used in this study was grown from U, Ru, and Si which were premelted, cleaned where applicable, and weighed. They were reacted and homogenized in an inert gas in a water-cooled-hearth arc furnace. The URu2Si2 crystal was grown by the Czochralski technique in a Reed-type Triarc furnace which has been modified to give a directly water-cooled hearth and seed rod. Ti gettered argon at 100 kPa was used as the chamber atmosphere. The growth rate was 20 mm/h. The rod was rotated at 9 rpm and the hearth at 60 rpm in the same direction.The crystal was cleaved perpendicular to the c axis and the reflectance of the cleavage plane was measured against a stainless-steel reference mirror. An accurate (±0.5%) value of the reflectance was obtained by coating the sample with Pb in situ in order to correct for differences between the position of the sample and reference in the optical path. 5 The reflectance was measured from 10 to 160 cm -1 with a Martin-Puplett-type polarizing interferometer and from 110 to 720 cm -1 wi...