Abstract: Cholinesterase activity is determined by means of the choline and/or acid obtained by the action of cholinesterase on a compound of the formula: ##STR1## [wherein A represents a hydrogen atom or a C.sub.1-5 alkyl or alkoxy group and B represents a group of the formula --R-COO-- (in which --R-- represents an alkenyl group with 2 to 6 carbon atoms); or A represents a hydroxy group in either or both o-positions of the benzene ring or a C.sub.1-5 alkyl group and B represents a group of the formula --COO--; X represents a stabilizing anion and n represents the charge on the anion]. The method of the invention ensures a high resistance to saturation, good stability of the compounds used as substrates, a conveniently low reaction rate and a high degree of accuracy.Certain of the compounds of formula I as defined above are novel compounds which are claimed per se.Examples of the method of the present invention are given and the preparation of the novel compounds is described and exemplified.