The aim of this study was to compare the degree of frequency separation that is required between tones to generate mismatch negativity (MMN) in a group of children with specific language impairment (SLI) and a comparison group, who had their temporal processing abilities assessed in a previous experiment. Using a 1000Hz standard (85%) and 1020, 1050, and 1100Hz deviant tones presented at inter-stimulus intervals (ISIs) of 200 and 400ms, MMN was compared in 19 children with SLI (13 males, six females, age range 7y 4mo-11y 10mo, mean age 9y 7mo [SD 1y 2mo]), and 19 comparison children (13 males, six females, age range 7y 3mo-11y 4mo, mean age 9y 5mo [SD 1y 3mo]). Temporal processing ability was assessed by the Auditory Fusion Test-Revised. Children with SLI who had poor temporal processing abilities generated a positive mismatch response (P-MMR) for 2% tone contrasts at 400ms ISI but MMN with larger contrasts. These children also generated stronger MMN than the comparison group at 200ms ISI for 2% contrasts. Children with SLI who had good temporal processing abilities generated only P-MMR in response to contrasts up to 10% for both ISIs. Some children with SLI show an inverse relationship between frequency discrimination and temporal processing. Furthermore, certain stimulus-related and biological criteria may need to be met for P-MMR to switch to MMN. © 2008 Mac Keith Press.