Cirrus (ice) clouds are often semitransparent (transmittance greater than zero, but less than one), which makes it difficult to measure fundamental cloud properties, such as albedo. This paper explores a way to measure the albedo of thick cirrus clouds (zero transmittance) at 3.9 ¿m using GOES Imager data. In brief, the 10.7 ¿m GOES data are used to separate the cirrus clouds into temperature classes. Then the measured 3.9 ¿m radiance is plotted against the cosine of the solar zenith angle. A theoretical relationship between radiance and solar zenith angle is used to extract the albedo. The result is that thick cirrus clouds have an albedo of 1.08% ¿ 0.05% at 3.9 ¿m. The albedo of thick cirrus is a step toward the measurement of the transmittance of thin cirrus.