In teaching and assessing pronunciation of English as an international lingua franca (ELF), intelligibility is more relevant than nativelikeness (Jenkins, 2015). As guidelines for intelligible ELF pronunciation, the Lingua Franca Core (LFC) syllabus (Jenkins, 2002) and relative functional load (FL) of phonemic contrasts (e.g., Catford, 1987) have been used (e.g., Jeong et al., 2018; Rahimi & Ruzrokh, 2016; Sewell, 2017).The paper examines phonemic details in the LFC and relative FL, based on the intelligibility of second language speakers’ phoneme realisation for Swedish university students. Using the perception of a group of Swedish youths for the study can be rationalised that they are known to have very high proficient English skills as a second language (Norrby, 2015). Speech data with IPA transcriptions were from the Speech Accent Archive (http://accent.gmu.edu/index.php), comprising nine speakers’ readings of the same text, whose first languages were Arabic, French, German, Japanese, Russian, Somali, Thai, Turkish and Urdu respectively. Each of seventy-five Swedish students taking university courses chose and transcribed one of the nine speakers in English orthography. Through comparing errors in the listeners’ transcriptions, their accounts, and the speakers’ segmental features deviating from either American or British English phoneme inventory, we firstly analysed whether, and to what extent such deviation affected intelligibility. From this analysis, some details of the LFC and relative FL were questioned. For example, while the LFC denotes that all consonants besides interdental fricatives need to be realised as in Standard American/British English, replacing some consonants with others, like plural marking /z/ with /s/ or alveolar /ɹ/ with uvular / ʁ/, did not compromise intelligibility. Likewise, while the ɔ/oʊ contrast is known to have high FL, replacing one with the other did not cause misunderstanding (e.g. realising ‘only’ as [ɔnli]). The findings suggest further scrutinising and developing the LFC and relative FL.