Two experiments were conducted to examine the processing of Korean compounds in relation to saisios. Saisios is a letter interposed between constituents when a phonological change takes place on the onset of the first syllable of the second constituent. This saisios rule is often violated by writers, resulting in many words having two spellings: one with saisios and the other without saisios. Among two spellings, some words are more familiar with saisios, some are usually spelled without saisios, and some are balanced. In Experiment 1 using the go/no-go lexical decision task, participants were asked to judge compounds with/without saisios. Saisios-dominant words (나뭇잎 > 나무잎) were responded faster when they appeared with saisios, whereas the opposite was true for words that usually appear without saisios (북엇국 < 북어국). In experiment 2, we presented participants compound words that were balanced on saisios. The results showed that words without saisios were responded faster than words with saisios. To summarize, the results of Experiment 1 and 2 were consistent with the APPLE model. Some problems related to the saisios rule were discussed in terms of reading process.