Previous research has identified linguistic differences within Welsh-English bilingual children from predominantly English homes and homes where both Welsh and English were spoken are found to lag behind L1 Welsh children from Welsh speaking households on measures of Grammatical Gender (Gathercole & Thomas, 2009); Plural Morphology (Thomas, Williams, Jones, Davies & Binks, 2014); and Welsh vocabulary (Rhys & Thomas, 2013). This thesis aimed to establish the factors that could influence whether 2L1 and L2 Welsh bilingual children eventually reach comparable attainment levels or ‘catch-up’ to that of their L1 Welsh peers in their command of Welsh. Three experiments assessed participants aged 12-13 and 16-17 from different language backgrounds on their knowledge of Grammatical Gender; Plural Morphology; Welsh vocabulary, and English vocabulary. The data obtained indicated that there was a ‘catch-up’ within the 12-13 participants on measures of vocabulary and Gender; however, L2 Welsh bilinguals still lagged behind on measures of Plural Morphology. The ‘catch-up’ was not seen within the older 16-17 participants. A fourth experiment investigated how the participants, use of Welsh, attitude towards Welsh, and their confidence using Welsh influenced their overall competence on these linguistic measures. Results revealed no correlation between attitude and participants success on each test. However, there was a correlation between the participants’ use of Welsh and their knowledge of Welsh, yet it did not contribute to higher scores. Confidence in using Welsh was revealed to be the sole predictive factor in participants’ success on each test.