Modal particles (MPs) like German bloß form a heterogeneous lexical category. One common property is that they do not contribute to the propositional meaning of a sentence, but rather display a Not-At-Issue (NAI) meaning. All of these words are ambiguous between the NAI meaning of the MP and an At-Issue (AI) meaning of a counterpart (e.g., focus particles, adverbs, and conjunctions). Unlike MPs, the counterpart typically affects truth conditions, like bloß as a focus particle with the translation ‘only’. So far, there has been little psycholinguistic research on the processing of MPs, the counterparts and their differing contributions to sentence meaning. We present the results of a corpus study where we measured the relative frequencies of both MP and counterpart readings (i.e., whether a specific word occurs more often as an MP or a counterpart) and a self-paced reading experiment on the processing of both meaning types. In our experiment, we varied MP and counterpart readings, and the position of the disambiguating region. We also examined the influence of the relative frequencies of both meaning types on the processing. The results point to processing differences between the NAI and the AI meaning of German MPs and their counterparts, suggesting that the two meanings are represented in different dimensions of meaning.