C. Fernandez-Pineda† and S. Velasco*,‡ †Departamento de Fisica Aplicada I (Termologia), Universidad Complutense, 28040 Madrid, Spain ‡Departamento de Fisica Aplicada, Universidad de Salamanca, 37008 Salamanca, Spain I the paper of Sandler and Woodcock one reads that the term “zeroth law” (of thermodynamics) first appears in the literature in 1939 in a book by Fowler and Guggenheim. This is also the opinion of others. However, other authors attribute this term to Fowler in 1931. This is an old controversy in thermodynamics literature. The allocation of the date of 1931 is due to Kestin. However, in the list of Fowler's publications compiled by Milne in 1945, no publications related to the zeroth law appear. The conjecture that Kestin is the first in attributing to Fowler the term “zeroth law” in 1931 is suggested by the Sommerfeld book Thermodynamics and Statistical Mechanics. In page 1 of this book one reads: “following a suggestion by R. H. Fowler, we shall refer to it as to the 'Zeroth Law' of thermodynamics”. The corresponding footnote is: “When given an account of the book of Thermodynamics of the great Indian astrophysicist M. N. Saha and his collaborators, B. N. Srivastaba, Allahabad 1931 and 1935”. The translator of the Sommerfeld book from German into English was Kestin. To check that the above note was not added by Kestin, we consulted the German edition not finding any addition to the English version from the German. The two editions, 1931 and 1935, of the book by Saha et al. correspond to two books with different titles. The 1931 edition is entitled A Text Book of Heat (including Kinetic Theory of Matter, Thermodynamics, Statistical Mechanics, and Theories of Thermal Ionization) (Allabahad: The Indian Press Ltd., 1931), while the 1935 edition is entitled A Treatise on Heat (including Kinetic Theory of Gases, Thermodynamics and Recent Advances in Statistical Thermodynamics) (Allabahad and Calcutta, The Indian Press Ltd., 1935). By searching for possible reviews of the books by Saha et al. in journals from the United Kingdom in the 1930s we found the two following accounts: 1. Advances in Heat Studies. Nature, Vol. 130, pp 794−795 of November 26, 1932, authored by N. M. B., not identified by the journal, without any reference to the zeroth law. 2. Heat a Mode of Motion: A Modern Version. Nature, Vol. 137, pp 554−556 of April 4, 1936, authored by R. H. F., the initials of Ralph Howard Fowler. It is in this review where the term “zeroth law” appears for the first time. Indeed, on page 555 it says: “A proper development of thermodynamics should, I am convinced, begin with the 'zeroeth' law of thermodynamics: There exists a variable called the empirical temperature which may be used in def ining the state of any system having the property that it has the same value for all parts of a system in thermal equilibrium” (italics in original).