The neutron diffusion length of graphite with an added 1/v absorber has been calculated for temperatures from 200 to 2000°K. At each temperature, the entire range of absorption for which a diffusion length exists has been examined. The largest such range, which occurs for 2000°K, extends from the absorption of pure graphite to a value corresponding to 7.3 b at 0.0253 eV. For all values of temperature and absorption, the diffusion length, L, in centimeters is given within 1% by where T is the absolute temperature and σ′ (b) is the absorption cross section at 0.0253 eV. The temperature-dependent functions α(T) and β(T) are given in Figs. 1 and 2. Very good agreement exists between the values implied by this formula and recent measurements. Each calculated value of L has been obtained as an eigenvalue of a P11 approximation to the transport equation. The neutron energy spectrum is obtained accurately as the eigenfunction in the calculation. This spectrum shows the characteristic discontinuity at the Bragg limit, previously known for beryllium, which varies in magnitude with the absorption. There is also a marked hardening of the spectrum with increasing absorption, which significantly affects the value of the diffusion length.