This study is concerned with a formal representation of auxiliation in four auxiliary constructions in Persian. These include the modal ‘ bayad’ construction, the future tense ‘ khastan’ construction, the aspectual ‘ dashtan’ construction, and the modal ‘ tavanestan’ construction. Conducted within a Role and Reference Grammar framework, the study explored the degree of grammaticalization in each construction utilizing three parameters of person/number inflection, tense-sensitivity, and contiguity. Each construction reacts differently with respect to these parameters to the extent that the auxiliary verb in the relevant construction receives a distinctive syntactic treatment in the layered structure of the clause. ‘ Bayad’ is taken to be a fully developed operator exclusively projected in the operator projection. ‘ Khastan’ , together with the lexical head, forms a complex nucleus demonstrating tenseinflection as well in the operator projection. ‘ Dashtan’ , constituting a non-predicating nucleus, stands in an ad-nuclear subordinate relation to the lexical head in the progressive construction. ‘ Tavanestan’ is the only modal verb within this quadripartite division that fails to constitute an auxiliary construction due to the fact that it predicates a separate argument structure of its own in the spirit of the least amount of semantic bleaching. These all attest to the fact that the syntactic projection of each auxiliary verb in the layered structure of the clause is a reflection of the degree of grammaticalization in the relevant auxiliary construction.