Abstract: The present invention relates to a novel reactive thiophosphate derivative of thia(dia)zole acetic acid which can be very effectively used in the preparation of .beta.-lactam antibiotics, and which is represented by the following general formula (I): ##STR1## in which R.sup.1 represents hydrogen or an amino-protecting group;R.sup.2 represents hydrogen, C.sub.1 -C.sub.4 alkyl, or --C(R.sup.a)(R.sup.b)CO.sub.2 R.sup.c, wherein R.sup.a and R.sup.b are identical or different from each other and represent hydrogen or C.sub.1 -C.sub.4 alkyl, or R.sup.a and R.sup.b together with a carbon atom to which they are bound can form a C.sub.3 -C.sub.7 cycloalkyl group and R.sup.c is hydrogen or a carboxy-protecting group;R.sup.3 represents C.sub.1 -C.sub.4 alkyl or phenyl or R.sup.3 together with an oxygen atom and a phosphorus atom to which it is bound can form a 5- or 6-membered heterocyclic ring; andQ represents N or CH, and to a process for preparing the same.