The subject of this study is how pupils in math disabilities communicate and reason during calculations involving fractional numbers. The study comprises eight informants, two from eigth grade (a boy and a girl, 13 to 14 years old) and six from ninth grade (three girls and three boys, 14 to 15 years old) who all were considered to have math disabilities. They were all interviewed alone about their reasoning when solving ten tasks involving fractions to see what knowledge and misconceptions the pupils showed. The conclusion of the study is that the pupils posessed procedural abilities, but were weaker in their concepual abilities. Most pupils in the study were uncertain about the meaning of nominator and denominator. Several of the pupils preferred to think in percent rather than fractions but they had problems in converting from percent to fractions and vice versa.