A perturbation method is employed to determine the thermal history of a biological organ frozen with a constant cooling rate at its outer surface. Solutions were obtained for the time history of the propagating phase front and the distribution of cooling rates within the organ. By means of a one-dimensional analysis, a new dimensionless group has been found to correlate the range of cooling rates in a frozen organ of complex shape. The analytical model has been confirmed by experimental measurements on a one-dimensional freezing system. The results of this treatment are useful in the design and analysis of experiments dealing with freezing in one dimension, particularly in the preservation of organs by freezing.