This technical report is applicable to the Raman gain efficiency measurement of a single-mode transmission optical fibre. It is useful in assessing the fibre's performance in Raman amplified transmission systems. This technical report describes a method that uses two unmodulated continuous waves to measure the Raman gain efficiency of a single-mode transmission optical fibre. This parameter assesses the fibre's efficiency at converting input pump power to information signal power. This second edition differs from the first in that in the previous edition, in the paragraph before Figure 2, there was an approximation of the relationship between wavelength and optical frequency that led to some inconsistencies in interlaboratory agreement. This approximation has been removed.