New methods are put forward to explain the numerical values of some useful bulk physical properties of amorphous copolyamides, in terms of parameters related to monomer structure. Polyamides based on adipic, tetramethylsuberic, iso- and tere-phthalic acids, and diamines including isophorone, xylylene, cyclohexane, hexamethylene and its trimethyl derivatives, methylnonane and dodecamethylene diamines were studied. ε-Caprolactam and 12-aminododecanoic acid were also used as comonomers. In Part I, an empirical rule is proposed, based on experimental observations which predicts whether a copolyamide has an amorphous or crystalline character. The rule is based on the individual stereochemical contributions of the constituent monomers to the overall polymer chain structure. A relationship between Vicat softening point and monomer composition is derived from experimental data, which seems to be generally applicable to all amorphous polyamides of the diacid/diamine type. Each monomer makes a molar contribution which has been determined experimentally for all the materials studied. An arbitrary set of simple structural rules has been devised which enables the molar contributions of monomers to be related to their chemical structure. This procedure provided a method of predicting the contributions of other monomers for which molar constants had not been measured, and was successfully tested for a limited number of materials. A modified relationship was obtained experimentally to explain the effect of amino-acids on Vicat softening point. In Part II, the relationships outlined in Part I are combined on the basis of experimental evidence to provide an empirical relationship between composition and impact strength. This relationship predicted the impact strengths of the majority of eighty copolyamides, of widely different chemical structure, with a reasonably good accuracy. Substantial inaccuracy occurred only when a large proportion of a long chain aliphatic monomer was present in the polyamide. Tentative correlations between tensile strength and carbon chain length were observed from a limited number of measurements which suggests that tensile strength may be a constitutive property. The main conclusions of this work are: (a) polyamides are naturally crystalline with high melting points if more than 80% of the monomer units are symmetrical; (b) Vicat softening point and tensile strength decrease linearly with increasing monomer chain length; the softening point is particularly affected by the presence of substituent groups; (c) amino-acids reduce symmetry, impact strength and Vicat softening points of copolyamides; (d) Charpy impact strength increases with the proportion of symmetric monomer units, the rigidity of the acid and flexibility of the diamine structures; (e) tensile strength and flexural modulus correlate with each other in copolyamides of diacids and diamines and both increase as the amount and chain length of aliphatic monomer is reduced; (f) by using the empirical relationships developed in this work, it has proved possible to formulate amorphous polyamides with outstanding combinations of physical properties, when compared with commercially available polymers.