A computerised cost estimating system, which can be used for making preliminary estimates of fixed capital investment of process plants, has been developed. The contituents of the system are (1) a cost calculating routine, COSMOD, compatible with existing modular executive programs, such as GEMCS, (2) an equipment data base, and (3) an equipment catalogue. The following are the significant characteristics of the cost estimating system.It has flexibility. It can be used as an independent cost estimating system, or as part of a modular executive-aided computer program. As part of an executive aided system, it may be used as a subroutine to a 'module* or 'unit computation', or as a 'module' or 'unit computation' itself. It will estimate the cost of one or more equipment but is capable of accepting costs generated external to itself. It requires relatively little computer space and time. The COSMOD program is concise and efficient. Its common block requirements in executive-aided systems is small. It adds little to the cost or time of running equipment design cases on the computer. It has access to a large quantity of data on equipment costs. Its data base contains cost information on nearly 300 different types of equipment, ensuring satisfactory fulfilment of its varying and often complex data requirements. The data are organised in a multiple record, random access file. This allows direct access to the equipment data of interest. It also allows easy addition, deletion or change to the data on file. An equipment catalogue is provided for the user. It provides information about the equipment whose cost data are available in COSMOD*s data file, and serves as a guide for the user in selecting and in providing data required by COSMOD. In conclusion, this work makes available, complete with cost estimating data and equipment catalogue, a preliminary capital cost estimating system, the absence of which has discouraged or impeded economic evaluations in plant design studies on the computer. / Thesis / Master of Engineering (ME)