The paper discusses the semantic description of the so-called discourse formulae, idiomatic
expressions used as the speaker's reactions in a dialogue. They are considered in the
framework of construction grammar, as a peripheral class of constructions with its specific
properties. A case study of two synonymous Russian discourse formulae ty čto and čto ty provides
an account of the components that need to be represented in the description of the constructions that belong to this class: namely, the characteristics of the speech act to which the
discourse formula reacts and the characteristics of the reaction itself. The analysis of entries
of the two constructions in the Russian National Corpus reveals that the range of meanings
that can be expressed by them includes: 1) negative assessment of the interlocutor's decision;
2) questioning the cause of the ongoing situation; 3) interrupting the interlocutor's action
(speech act) or state of mind (i.e., consolation and reasoning); 4) surprise, and 5) negation.
These results give an estimate of the set of meanings that may be attributed to other discourse
formulae. A quantitative corpus analysis of the use of these two constructions is also conducted,
showing that their distribution between the defined types of the given contexts is not
identical: in particular, čto ty tends to express negation and is rarely used to express negative
assessment. These differences are discussed in the light of the diachronic change of ty čto and
čto ty, suggesting different patterns of their pragmaticalization.