Strange to say, no suitable dry-powder formula for acid hardening fixing bath has ever been published in literatures except the case of Woosley and Pankhurst of Ilford Ltd., in which sodium bisulfate (or sodium hydrogen sulfate) was used as sole solid acid instead of liquid (acetic) acid in ordinary solution formulae. It seems usual to use bisulfite or metabisulfite as solid acid in conjunction with sodium acetate in making up dry-powder formulae, because acetic acid is usually used with sodium sulfite in ordinary solution formulae. And it is found that useful dry-powder formulae can always be prepared from almost all existing solution formulae using some combinations of the following chemical equations; Na₂SO₃+CH₃COOH=NaHSO₃+CH₃COONa (1) 2Na₂SO₃+2CH₃COOH=Na₂S₂O₅+2CH₃COONa+H₂O (2) Na₂SO₄+CH₃COOH=NaHSO₄+CH₃COONa (3) NaBO₂2H₂O+CH₃COOH=H₃BO₃+CH₃COONa+H₂O (4) For examples author's dry-powder formula of PF-6 is prepared from famous Kodak's solution formula of F-6 by combination of equations (2) and (4), and this is the best formula he recommends, because it is very stable and can be made up in single dry-powder package by only mixing the ingradients in powder form. Also author's PF-5 is derived from F-5 formula by the combination of equations (2) and (3). Other examples are also shown in Chapter 6 which includes the inverted case of obtaining a solution formula from existing dry-powder formula such as Formula D of Woosley and Pankhurst. Author's formula of PFA-6 is derived from PF-6 by only replacing sodium thiosulfate (150 g/ l ) in the formula with the (50: 50) mixture of ammonium and sodium thiosulfate (200 g/ l ). This is satisfactory rapid-acting and has a sufficient hardening property. Both PF-6 and PFA-6 can be satisfactorily used as a Stop-Fix bath for color-papers. All reference formulae are shown in the last table.