For Ma subsystem in a complex monomolecular reaction system containing irreversible reactions

the character of the only observed straight line reaction path in two dimensional coordinates is observed. A mathematical expression of ray vector as an implicit function of rate constants is derived. Taking advantage of the expression as a set of linear equality constraints

the relative rate constant matrix of hydroisomerization of C8-aromat-ics is estimated by means of flexible tolerance polyhedron. It is proved that the uncertainty of curve-fitting can be reduced because of these constraints. It provides an effective resort for parameter estimation of Ma monomolecular subsystem.