The pK values of some weak acids were measured using the hydrogen electrode in liquid ammonia containing 0.1Mpotassium iodide supporting electrolyte at −60°C. The results showed that acetone (pK = 22.5), aniline (pK = 28.8), and 4-chloroaniline (pK = 27.4) were weaker acids than water (pK = 18.9), while thiophenol, diphenyl thiourea, thiocarbohydrazide (pK<2), phenol (pK = 3.5), phenylthio-urea (pK = 3.8), thioacetamide (pK = 4.6), thiosemicarba-zide (pK1= 4.7; pK2= 8.5), thiosinamine (pK = 6.6), diphenylurea (pK = 7.0), diphenylsemicarbazide (pK = 7.6), diphenylguanidine (pK = 10.7), and diphenylcarbazone (9.5<pK<11) were stronger than water.