Aminoacyl-transfer RNAs contain four standardized units: amino acids, an invariant 3′-terminal CCA, trinucleotide anticodons and tRNA bodies. The degree of interchangeability of the three variable modules is poorly understood, despite its role in evolution and the engineering of translation to incorporate unnatural amino acids. Here, a purified translation system is used to investigate effects of various module swaps on the efficiency of multiple ribosomal incorporations of unnatural aminoacyl-tRNA substrates per peptide product. The yields of products containing three to five adjacent L-amino acids with unnatural side chains are low and cannot be improved by optimization or explained simply by any single factor tested. Though combinations of modules that allow quantitative single unnatural incorporations are found readily, finding combinations that enable efficient synthesis of products containing multiple unnatural amino acids is challenging. This implies that assaying multiple, as opposed to single, incorporations per product is a more stringent assay of substrate activity. The unpredictability of most results illustrates the multifactorial nature of substrate recognition and the value of synthetic biology for testing our understanding of translation. Data indicate that the degree of interchangeability of the modules of aminoacyl-tRNAs is low.