Tandem repeats of various trinucleotide motifs are present in the human transcriptome, but the functions of these regular sequences, which likely depend on the structures they form, are still poorly understood. To gain new insight into the structural and functional properties of triplet repeats in RNA, we have performed a biochemical structural analysis of the complete set of triplet repeat transcripts, each composed of a single sequence repeated 17 times. We show that these transcripts fall into four structural classes. The repeated CAA, UUG, AAG, CUU, CCU, CCA, and UAA motifs did not form any higher order structure under any analyzed conditions. The CAU, CUA, UUA, AUG, and UAG repeats are ordered according to their increasing tendency to form semistable hairpins. The repeated CGA, CGU, and all CNG motifs form fairly stable hairpins, whereas AGG and UGG repeats fold into stable G-quadruplexes. The triplet repeats that formed the most stable structures were characterized further by biophysical methods. UV-monitored structure melting revealed that CGG and CCG repeats form, respectively, the most and least stable hairpins of all CNG repeats. Circular dichroism spectra showed that the AGG and UGG repeat quadruplexes are formed by parallel RNA strands. Furthermore, we demonstrated that the different susceptibility of various triplet repeat transcripts to serum nucleases can be explained by the sequence and structural features of the tested RNAs. The results of this study provide a comprehensive structural foundation for the functional analysis of triplet repeats in transcripts.