diff --git a/src/libcollections/vec_deque.rs b/src/libcollections/vec_deque.rs
index 2aeb303068bd..117b3544f026 100644
--- a/src/libcollections/vec_deque.rs
+++ b/src/libcollections/vec_deque.rs
@@ -108,7 +108,7 @@ impl<T> VecDeque<T> {
         ptr::write(self.ptr.offset(off as isize), t);
     }
 
-    /// Returns true iff the buffer is at capacity
+    /// Returns true if and only if the buffer is at capacity
     #[inline]
     fn is_full(&self) -> bool { self.cap - self.len() == 1 }
 
diff --git a/src/libcore/num/mod.rs b/src/libcore/num/mod.rs
index fd5ef4b1cccc..3fd3d4a42b20 100644
--- a/src/libcore/num/mod.rs
+++ b/src/libcore/num/mod.rs
@@ -1126,7 +1126,7 @@ macro_rules! uint_impl {
             acc
         }
 
-        /// Returns `true` iff `self == 2^k` for some `k`.
+        /// Returns `true` if and only if `self == 2^k` for some `k`.
         #[stable(feature = "rust1", since = "1.0.0")]
         #[inline]
         pub fn is_power_of_two(self) -> bool {