AbstractSolubility data expressed as mole ratio,xRH(moles of RH per mole of liquidS) have been measured for propane andn‐butane in a number of type‐representative organic liquids,S, at different temperatures,t°C, and pressures,pRH≤ 760 mm. These data, and those for methane, ethane and radon, afford support in the development of the essential pattern of data for all gases,A, and all liquids,S.This pattern indicates that the first factor determining the solubility of any gas,A, in any liquid,S, is its tendency to condense to a liquid at the operational temperature,t°C (e.g. 0 °C to 25 °C), shown by its vapour pressurep°Aover liquidA(or its equivalent for a gas above the critical temperature) att°C. Sincep°Aemerges from the b.p. ofAat 1 atm, and b.p. data are readily seen in text‐books, the b.p. may be taken as an initial index of the first factor. In theNA–pAdiagram for a given gasAand different liquids,S, at a fixedt°C,NAbeing the mole fraction ofA, the straight line joiningp°AwithpA= 0 is herein called the reference line, theR‐line, ofA; it is a real property ofA, and it represents the plot ofNA–pAfor a hypothetical liquid,S, having in effect an overall inter‐molecular structure equivalent to liquidA, in imagination,Aitself. TheNA–pAplot for another liquid,S, involves a second and a third factor, the second being the intermolecular structure ofS, in a dynamic sense, and the third, theinevitableinteraction ofAwithS.These factors are deemed to emerge from changes in electron density, designated for the present as acid‐base functions. The spread ofxRHvalues is indicated byxPrH= 0.322,xBuH= 5.25, for di‐n‐octyl ether;xPrH= about 0.000042,xBuH= about 0.000040, for