[en] The ion yields and energy distributions of C^- and C₂^- ions sputtered from a graphite target by a 20 keV Cs⁺ beam, have been measured in the temperature range 80-880K. Assuming a velocity-dependent ionisation probability, the measured changes in the position of the peak of the energy distribution have been used to calculate the changes in the work-function of the emitting surface. It is concluded that the work-function reaches a broad minimum at approximately 500 K increasing thereafter by about 1.3 eV at the maximum achievable temperature in the experiment; this increase is accompanied by a corresponding decrease in the negative ion current by about a factor of 6. (author)